Holy cow, we ask and it happens! hahahahah!
This does open up stuff and I don’t disagree with your assessment that although it would be funny, c3po did have his mind erased and wouldn’t really know anything about the resistance or his role. Of course, you can assume he gets PARTIALLY informed at some point, but not the whole picture.
On the other hand, if we are focusing on the Jannah scene, he has gone through flying to a destroyer, Poe and crew shoots down storm troopers and they’ve flown to the DSII wreckage finding the wayfinder. So he would at least get the idea, or some explanation on who they are and what they are doing.
Hmmm it does make you think regardless.