So my question is, if I have 3 Xenagos, God of Revels out (1 original and 2 non-legendary copies, thanks to Helm of the Host), how do each of their combat abilities work? For example, if I have a 5/5 that I target with the original Xenagos to make it a 10/10, do the other two kick in separately making it a 20/20 and then a 40/40? Or do all three take effect at once and I just get the 10/10?
"At the beginning of combat on your turn", the triggered ability of each Xenagos will trigger at the same time (C.R. 603.2). Since they're separate abilities, they can target the same creature you control or different ones (C.R. 603.3d, 601.2c). However, the triggered abilities resolve one at a time (each time after all players pass in a row), so that they can work off the power determined by previous ones (C.R. 116.4).
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules