For those who do not know Greek, simply skip to the end.
Here’s the Greek (with what interests me most in bold):
Λέγω γὰρ διὰ τῆς χάριτος τῆς δοθείσης μοι παντὶ τῷ ὄντι ἐν ὑμῖν μὴ ὑπερφρονεῖν παρ᾽ ὃ δεῖ φρονεῖν ἀλλὰ φρονεῖν εἰς τὸ σωφρονεῖν, ἑκάστῳ ὡς ὁ θεὸς ἐμέρισεν μέτρον πίστεως. (Rom 12:3 BGT)
Upon first glance the obvious translation/meaning is, “For I am telling all of you, through the grace which was given to me, do not be thinking, with reference to yourselves, more highly than it is necessary to think, but rather think [with reference to yourselves] in order to be of sound mind; each one as God has given a measure of faith. (Romans 12:3 Geoff Translation)
Here are come common translations of this verse:
For I say, through the grace given unto me, to every man that is among you, not to think of himself more highly than he ought to think; but to think soberly, according as God hath dealt to every man the measure of faith. (Rom 12:3 KJV)
For by the grace given to me I say to everyone among you not to think of himself more highly than he ought to think, but to think with sober judgment, each according to the measure of faith that God has assigned. (Rom 12:3 ESV)
For by the grace given to me I say to everyone who is among you not to think more highly of yourself than what one ought to think, but to think [sensibly], as God has apportioned a measure of faith to each one. (Rom 12:3 Lexham English Bible)
For by the grace given to me I ask every one of you not to think of yourself more highly than you should think, rather to think of yourself with sober judgment on the measure of faith that God has assigned each of you. (Rom 12:3 International Standard Version)
For by the grace given to me I say to every one of you not to think more highly of yourself than you ought to think, but to think with sober discernment, as God has distributed to each of you a measure of faith.
(Rom 12:3 NET)
Every translation takes the Greek preposition εις to mean “with” and they do not note the force of the stative (verb about a state of being), instead they make it purely adverbial with the preposition (translated, it seems in a bizarre way). It would seem that Paul is actually contrasting two different ways of showing self-regard in relationship to the gifts God has given an individual. You can think of yourself too highly or you can think of yourself specifically for the purpose of having a sound mind about your abilities (upon which Paul is about to elaborate). This idea not only makes the most rhetorical sense: Do not be puffed up about yourselves, instead think about yourselves in a realistic way to determine how God has best gifted each of you to serve in the church. This idea also makes the most syntactical sense.
Now, I’m going against a giant trend in translations here, but εις rarely means ‘with’ and not ever that I can think of with an infinitive. But εις το + [infinitive] often connotes purpose.
Paul wants Christians to think about themselves so that they can judge their own capacities soundly. In this way the next five verses of Romans can be obeyed.