Jeff, I wanted to add a side note to my response. yes, having just 2 yellows will cause havoc sooner than later. as long as the tangs are different body styles and colors, you should not have much issue with them. for instance, the yellows are of the sailfin type while the powder is totally different shaped. also, yellows are notorious for being "junkyard dogs" I had one that would attack everythign entering the tank no matter what it was. he was since traded at the critter for a Purple, which is one of my favs. I have had much better success out of this one. I must have gotten a docile one. it has been at least a year since I got my purple and last month I added a kole. Very different color and body style. we had a few little fights, but after 1 overnight, peace prevailed. you may wish to consider a bit of variety if you want 3 of them. say maybe the yellow, a Naso and the Powder blue.
I so wanted a powder blue, but they dont really ship well and it is hard to get a healthy specimen, or at least I have tried. also the Powder is very succeptable to infection , and can be extremely mean. I did so much research on them and I still would kill for one. your 120 should give it plenty of room to roam about and if you are able to get a good one and place it in with 2 other healthy tangs at the same time, you should be in good shape. just make sure they are some of the last fish added.
and yes, they do produce alot of waste.