The yellow and the purple are almost identical in body shape. That's generally a no-go with surgeonfish. However, it's a fish by fish kinda thing, there's exceptions to every rule. Personally I'm a little leery of testing for the exception though. It's a real PITA to get a fish out of the tank when another fish is trying to attack it and you and everything's darting around in the rocks. Orange shoulders are very peaceful, and you could even have a pair in the same 180 gallon tank. They are naturally one of the only habitually schooling tangs. I would go with the purple, the orange, and a kole tang, myself. That way you have a diatom scrubber, a hair algae muncher, a red algae muncher, and a green algae muncher. They are all dissimilar in looks, and as the kole tang only reaches around 5-7", you shouldn't be stressing your bioload or swimming space too much. Remember, tangs are high oxygen level dependant, and if you have a power outage, you need to be sure they won't suffocate. Three big tangs in even a 180, IMO, is pushing it. Much better to have two big and one little. And the koles are really mellow, shy, and very handy. They are in truth one of my favorites. Think, with a 180, you could have the above trio of tangs, all very different in looks and as wide a spectrum of feeders as you can get in the same band of specimens, and then you'd still be able to have a nice selection of smaller scavengers, like a small school of chromis, or clown gobies, a few pairs of dart fish, maybe even some jawfish if you have the proper substrate. You could go with a indo-pacific themed tank, with your tangs being the big visible active fish, and your smaller shyer species being the seldom glimpsed and much coveted attention getters. Although, if you plan on having dragonets or sand gobies in the future, be sure not to stock any competitors. The ideal set up, for a true reeflike biotope, would be to have a few small slow sand sifters, some active mid level eaters to catch the food in your water column, (chromis, anthias, fairy wrasses, any small fish that won't pick on your rocks,) and then your big herbivores. Or you could get even more micro about it, make it species specific. But if you want multiple kinds of tangs, you've pretty well cut out that species specific part.