The yellow is ususally on of the last tangs added when more than one. Be careful. Mine is very territorial and beats up on a foxface rabbitfish - prolly 'cause its yellow too and is flat.
I am returning the foxface, removing the yellow to the QT
for 3 weeks timeout, re-arranging the rocks, and adding a yellow-eyed kole.
Is your tank old enuf to sustain the bio-load increase? I wouldn't do a third, but if you do, I'd think the new tang would have to be bigger than the yellow also. Again, back to the sustainable bio-load question. Also there's the issue of swimming room for 3 and the stress it may cause down the road - or immediately when it gets tight.