Depends if the owner wants to risk it or not.
I have had for 3 months...3 ngt (7, 3.5 and 3 inch), together with other Indos, various sizes. Bit of a risk, but I think the indos keep the bigger ngt under control. Bout a week ago, the bigger ngt got beaten up bad, and it wasnt by the smaller ngt's.
Make sure you have loads of other tigers to spread aggression and set up a QT tank just in case u need to seperate, thats what i did. Ill probably sell a couple off in the near future.
I very much doubt, 3 similar sized ngt will work (big or small)
I very much doubt, 2 similar sized ngt will work
I very much doubt mutiple NGTS are a good idea long-term. But it can be done short- term.