Given that those tangs each reach 14+ inches in their natual lifespans, an 18 inch tank is considered by many to be far too small for one of them, let alone three.. Personally, I wouldn't see much harm in housing these tangs in that kind of space for thier juvenile lives, but after about 3 inches, I'd be upgrading to a 24" deep tank, preferrably a 180 at the minimum..
Now, I know that is opposed to the wisdom you'll hear at many of our local fish stores, but if you do some research, you'll find that long term, it's better for the fish to be housed in a setting that will allow them at least the illusion of freedom to stretch thier fins as it were..
Aaron Whitaker