Hopefully a quick and simple one.

If I’m adding 3 black dice from a weapon with impact 3 to a pool of shots does impact translate to the whole pool? So if it’s 5 white dice 3 black do I convert any 3 hits to crits? Or do they have to be on the black dice?

Any 3 hits. The Impact Keyword effects the whole pool of dice, not just those relating to the weapon used.