I don't think Romans 12:3 is saying that every man born has been given a measure of faith. I think in that context it is referring to every man that was among them as a local assembly of born again Christians learning how to operated in the gifts of the Spirit.
3For 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.
4For as we have many members in one body, and all members have not the same office:
5So we,
being many, are one body in Christ, and every one members one of another.
6Having then gifts differing according to the grace that is given to us, whether prophecy,
let us prophesy according to the proportion of faith;
7Or ministry,
let us wait on
our ministering: or he that teacheth, on teaching;
8Or he that exhorteth, on exhortation: he that giveth,
let him do it with simplicity; he that ruleth, with diligence; he that sheweth mercy, with cheerfulness.
This "measure of faith" being applied to how they operate in their individual gifts. This is the context and it should not be used to apply to the idea that every person born has been given a measure of faith to operate in a spiritual gift as the ones listed. It applies to born again Christian who have the HOLY SPIRIT in them. To each of these has been given a measure of faith according to their gift.
Also to confirm that this is the correct interpretation and not that every man even born has been given a measure of faith for salvation we have the following verse.
2 Thess 3:2
And that we may be delivered from unreasonable and wicked men:
for all men have not faith.
So not everyman has faith and we know that to be true. Can all men choose to have faith, I believe they can. But not all do. There is no mystical "measure of faith" that they carry about unused. It is not faith unless you believe.
As to those of us in the body of Christ who have these spiritual gifts by the Holy Spirit after we are born again we have to have faith to operate in that gift and to each of
us has been given the measure of faith according to our gift. If you have the gift to teach you have been given faith to teach and you know that and are biting at the bit to teach because you have faith to release that gift in you. But you also know not to attempt another gift that the Spirit is not stirring your faith to attempt. In other words your measure of faith helps you to "stay in your lane" and not think too highly of yourself and attempt gifts that you have not been called by the Spirit to do.