I've been in this hobby for about 10 years now and I've successfully kept many tangs in a 55 gallon for over 3 years and I haven't had any problems.
How many tangs and for how long for each? Tangs can live 10-15years and 3yrs just is not a testament to success, especially if their growth rate has not been on par with what it should be. Most tangs as juveniles double in size every 6-8 months if healthy, then as they mature their growth slows. If the size of these fish is still that of juveniles where they can be comfortably housed in a 55g for three years then they have not grown in a manner that healthy fish should grow.
As long as you have good flow for aeration for oxygen and a strong skimmer you'll be fine.
Aggression and swimming space also play a major role. Tangs are active fish that use the entire length of large tanks to swim and graze and defend. T o cut that down and add more tangs to that is not something that is taking the well being of the fish into consideration.
I've seen many people have yellow tangs in 40 gallons and smaller.
Unfortunately so have I, and for every one person that provides a healthy environment there are ten more that within a period of months have killed that tang by insufficient care.
I now have a juvenile hippo tang in a fluval edge 6 gallon and he is fine.
Define fine, not dead, not at the moment covered in ich? By the time substrate and decor are added there is less then 5g of water volume in the 6g edge. How long can a fish that grows to a foot in length be properly provided for in that environment?
Now some may disagree with the way I've done it but that's what this hobby is. There's different ways to do something and they may work and they may not. It's a great big experiment that teaches you a lot.
There certainly is more than one way to be successful, but success should be defined more by satisfying the fish's needs and it growing, and developing in ways where it can stake out and defend a territory, show proper color, be of proper dimensions, not by "I didnt kill it so I have succeeded".
Experimentation and trial and error have done a lot for our hobby, but ultimately we are caring for living creatures. We have a responsibility to care for them to the best of our abilities not"lets see how small of a tank I can keep a tang in" and if we cant at least plan out what we will do if a fish outgrows what we can provide for it than we really need to reconsider keeping it in the first place.
I agree that there are situations where a tang that for most people will not work in a 55g will work for somebody that is more dedicated and pays greater attention or puts in more work, but to do that for someone that is looking for ideas and has not indicated that they are willing to do extra is not in the best interest of the op