I know that Gen 1 and 2 are, but are the Gen 3 Synths?

They might have implants that provide a resistance to it.

It is pretty safe to assume they are not since they are mostly organic almost indistinguishable from human(to a degree where even standard medical tests wont see a diference-thats ingame info). So i wud say its about 99,9% sure they are not more rad resistant than any averige human.

Gen 1 an 2s are basicly robots, so ofocurse radiation wont affect them.