The main problem as mentioned is the tank is too small for three. The naso can get pretty big. If you did not have the naso you could try the purple and yellow. There are plenty of tanks out there with purple and yellows together and powder blues with achilles, and purples or yellows with sailfins. Most people telling you you cant mix same shape tangs are not speaking from experience or what they have seen but rather what they have read. The keys to mixing tangs is a large tank, plenty of rock work, adding the tangs at the same time and keep them close to the same body size so most confrontations turn into stand offs rather then all out brawls.The truth is all tangs will fight at first if introduced at different times or one is much larger then the others.