We had three in a 75 gallon FOWLR that got along fine.
Yellow tang, Pacific Blue tang, and a Purple tang.
They all started out fairly small in the tank, and we knew that at some point we would need to either upgrade the tank or re-home them to someone with a larger tank.
They got along for several years with zero fighting and they were growing and looked unstressed and healthy. They were about (guessing) 5-6 inches long when we had a heat issue in our house (programable thermostat failed and cranked the heat on non-stop over a 3 day weekend where we were not home). We lost everything but some snails for livestock.
I wonder how long hey would have gotten along before they would have just been too big for the tank. the key may have been that they were well fed, and never had to compete for anything.
Now I am back after a few years looking to get back into the hobby and we would love to have a few tangs again, but in a larger tank. I wonder if we will be as lucky a seond time around or not.