Yep, I agree with Sheryl. If they're all getting along fine then I'd only consider using one for 'special occasions' like visitors staying over, fireworks, a new introduction etc
I do have one plugged in permanently but that's only because one of my cats is overly sensitive and a bit skittish/nervy at times. The Feliway really helps him and he's so much more confident with it plugged in 24/7. I don't
need to use one ie there's no terrotorial issues between them (they
each other) and no inappropriate peeing etc