It's species dependant, of course. some, like veileds, do very well in trios if conditions are right. Others, like some of the montane species, seem to do very poorly if kept completely isolated.
The key is space. One male veield wold be perfectly fine in a raised cage, about 2'x2'x3' high. You add a female to the mix, and you're looking at something 5-10 times that size - maybe more.
My male deremensis seemed to go nuts when housed out of visual contact from females.
When housed together, or at the very least near each other without a total visual barrier, they stopped roaming altogether.
Sometimes, housing them together is possible in relativly small cage - such as the case with breeding season deremensis. However, when breeding season is ending, and the females change their mood, you're going to need another cage. Strange thing is they might be ok for 6 months before she starts to let him know he's got to go.
It's really hard to know when things are good, acceptable, unacceptable or just plain dangerous. It's hard even with years of experience.
the "safe" answer is to say don't do it - after all, if you're new to the hobby, it's a very bad idea to try stuff like that. very bad. Things'll end up hurt or dead.
The main factor, is of course, space. Some species need more space than others, some individuals need more space than others. One male veiled I had needed a 12'x8'x8' outdoor cage to be housed with females. He was just too aggressive.
After a few months in the cage with two females, he learned to respect their visual cues. A smaller cage would have worked then - but not before.