You don't need to worry about adds in the last phase, place your mocking banner between Garrosh and ranged players (who should be loosely stacked/dodging swirls during Emp. Whirling Corruption), and have everybody full nuke on the boss. The
'beating' you get off the adds (if you can call it that >.>) adds some nice Vengeance to increase your dps on Garrosh. I tend to kill one of the adds so the others get buffed for even more Vengeance.
Biggest problem in phase 3 is the mind control IMO, Ring of Peace/Solar Beam still work even though the adds are meant to be 'unsilenceable' (the interrupt part of the abilities kicks in).
Hope this helps
GL!