Citadark Isle Shadow Salamence .

what is a xk.3 file and how to use it?
for Gen 3 games or gamecube files?

37 minutes ago, King Impoleon said:

what is a xk.3 file and how to use it?
for Gen 3 games or gamecube files?

Just drag it into PKHeX. It'll auto convert to whichever game you're playing.

It's natively Gen3 GameCube format (has Shadow data), but will auto purify if used outside those games. (I think it could only be traded over when purified)

This salamance can be shiny?

No Shadow Pokemon from XD can be shiny.