assuming that it is a standard 75g it's probably around 4 feet long 18 inches wide. If you keep one black Arowana in that it should outgrow it in probably a year from 5" if you keep 2 they will most likely end up killing eachother but if you keep 3 5" blacks in a 75g I wouldn't keep them in there for more then a few months. never under estimate how fast these guys will grow lol
One is the only guaranteed number to work. Ive seen just as many successful pairings as trios. Along with many failed trios. There is no predictable sure thing with multiple SA arowana. The key to attempting multiple arowana is having the means to separately house them quickly if things go wrong. I say if you can house two black aro separately, then give the pair a try. Realize you are going to eventually need more then a 125g tank to keep the pair in if it works.