Hi experts! I'd appreciate your help with a doubt about prof Charles.

I am trying to figure out if I equip prof with a sword from XoS, could I do B/C/F trough my fellow mutants?

His rally says:

RALLY (5): Opposing Attack Rolls. FREE: Remove one of Professor X's RALLY dice to choose a friendly character with a shared keyword within range and line of fire. This turn, Professor X may count range and draw lines of fire from the chosen character and uses his printed combat values when doing so

A second part of the question is, in case I can use B/C/F, would the printed damage be overriden by BCF, or max damage would be his natural 3 damage?

Thanks for the help!

No. Close attacks require adjacency, and neither of the two effects granted by the rally die (counting range and drawing line of fire) override this.

If you want just to borrow another character's stats to make a close attacks with Chuck:

BLADES/CLAWS/FANGS When this character makes a close attack against a
single target and hits, you may roll a d6. If you do, deal damage equal to the result instead of normal damage. Minimum result is this character’s printed damage value -1.

B/F/C change Damage Dealt (which is not a stat) and not Damage (which is a stat).
The minimal damage will still be Chuck's Damage -1 (since the power use printed damage not current damage)