Just for fun let's see how Philo - another near contemporary - approaches this passage:
God therefore here is propitiated by three different kinds of repentance, by the aforesaid beasts, or by the birds, or by the while flour, according, in short, to the ability of him who is being purified and who repents. For small offences do not require great purifications, nor are small purifications fit for great crimes; but they should be equal, and similar, and in due proportion. (236) It is worth while, therefore, to examine what is meant by this purification which may be accomplished in three ways. Now it may almost be said that both offences and good actions are perceived to exist in three things; in intention, or in words, or in actions. On which account Moses, teaching in his hortatory admonitions that the attainment of good is not impossible nor even very difficult, says, "It is not necessary to soar up to heaven, nor to go to the borders of the earth and sea, for the attainment of it, but it is near, yea, and very Near." And then in a subsequent passage he shows it all but to the naked eye as one may say, where he says, "Every action is in thy mouth, or in thy heart, or in thy Hands:" meaning under this symbolical expression, in thy words, or in they designs, or in thy actions. For he means that human happiness consists in wise design, and good language, and righteous actions, just as the unhappiness arises from the contrary course. For both well-doing and wrong-doing exist in the same regions, in the heart, or in the mouth, or in the hand; for some persons decide in the most righteous, and sagacious manner, some speak most excellently, some do only what ought to be done: again, of the three sources of error the most unimportant is to design to do what ought not to be done, the most grievous is to do what is iniquitous, the middle evil is to speak improperly. But it often happens that even what is least important is the most difficult to be removed; for it is very hard to bring an agitated state of the soul to tranquillity; and one may more easily check the impetuosity of a torrent than the perversion of the soul which is hurrying in a wrong direction, without restraint. For innumerable notions coming one upon the other like the waves of a stormy sea, bearing everything along with them, and throwing everything into confusion, overturn the whole soul with irresistible violence. Therefore the most excellent, and most perfect kind of purification is this, not to admit into one's mind any improper notions, but to regulate it in peace and obedience to law, the ruler of which principles is justice. The next kind is, not to offend in one's language either by speaking falsely, or by swearing falsely, or by deceiving, or by practicing sophistry, or by laying false informations; or, in short, by letting loose one's mouth and tongue to the injury of any one, as it is better to put a bridle and an insuperable chain on those members.
But why it is a more grievous offence to say what is wrong than only to think it, is very easy to see. For some times a person thinks without any deliberate previous intention of so thinking, but inconsiderately: for he is compelled to admit ideas in his mind which he does not wish to admit; and nothing which is involuntary is blameable: (242) but a man speaks intentionally, so that if he utters words which are not proper he is unhappy and is committing offence, since he does not even by chance choose to say anything that is proper, and it would be more for his advantage to adopt that safest expedient of silence: and, in the second place, anyone who is not silent can be silent if he pleases. (243) But what is even a still more grievous offence than speaking wrongly, is unjust action. For the word, as it is said, is the shadow of the deed; and how can an injurious deed help being more mischievous than a shadow of the same character? On this account Moses released the mind, even when it yielded to many involuntary perversions and errors, from accusations and from penalties, thinking that it was rather acted upon by notions which forced their way into it, than was itself acting. But whatever goes out through the mouth that he makes the utterer responsible for and brings him before the tribunal, since the act of speaking is one which is in our own power. (244) But the investigation to which words are subject is a much more moderate one, and that with which words are united is a more vigorous one. For he imposes severe punishments on those who commit gross offences, and who carry out in action, and utter with hasty tongues what they have been designed in their unjust minds.
Therefore he has called the purifying victims which are to be offered up for the three offenders, the mind, speech, and the action, a sheep, and a pair of turtle doves or pigeons, and the tenth part of a sacred measure of fine flour; thinking it fit that the mind should be purified by a sheep, the speech by winged creatures, and the action by fine flour: Why is this? (246) Because, as the mind is the most excellent thing in us, so also is the sheep the most excellent among irrational animals, inasmuch as it is most gentle, and also as it gives forth a yearly produce in its fleece, for the use and also for the ornament of mankind. For clothes keep off all injury from both cold and heat, and also they conceal the unmentionable parts of nature, and in this way they are an ornament to those who use them: (247) therefore the sheep, as being the most excellent of animals, is a symbol of the purification of the most excellent part of man, the mind. And birds are an emblem of the purification of speech: for speech is a light thing, and winged by nature, flying and penetrating in every direction more swiftly than an arrow. For what is once said can never be re-called; but being borne abroad, and running on with great swiftness, it strikes the ears and penetrates every sense of hearing, resounding loudly: but speech is of two kinds, one true and the other false; (248) on which account it appears to me to be here compared to a pair of turtle doves or young pigeons: and of these birds one he says is to be looked upon as a sin offering, since the speech which is true is wholly and in all respects sacred and perfect, but that which is false is very wrong and requires correction. (249) Again, as I have already said, fine flour is a symbol of the purification of activity, but it is sorted from the commoner sort by the hands of the bakers, who make the business their study. On which account the law says, "And the priest having taken an entire handful, shall place it on the altar as a memorial of them," by the word handful, indicating both the endeavor and the action. (250) And he speaks with exceeding accuracy with respect to the sheep, when he says, "And if his hand be not strong enough to supply a sheep;" but with respect to the birds he says, "And if he cannot find a bird." Why is this? Because it is a sign of very great strength and of excessive power, to get rid of the errors of the mind: but it does not require any great strength, to check the errors of words; (251) for, as I have said already, silence is a remedy for all the offences that can be committed by the voice, and every one may easily practise silence; but yet, by reason of their chattering habits and want of moderation in their language, many people cannot find out how to impose a limitation on their speech.
So let's break this down further. In the LXX:
ἔστιν σου ἐγγὺς τὸ ῥῆμα σφόδρα ἐν τῷ στόματί σου καὶ ἐν τῇ καρδίᾳ σου καὶ ἐν ταῖς χερσίν σου αὐτὸ ποιεῖν
The word is very near thee, in thy mouth, and in thine heart, and in thine hands to do it.
Philo and Clement take the threefold division - mouth, heart, hands - to relate to three types of sacrifice and three kinds of human actions. Paul doesn't include 'hands' in his citation of Deut 30.14 which is odd. There is no reference to hands in the Hebrew. But Marqe knows it.