I can't give you a direct answer, but since no responses, so far, here goes...
I've never bothered to vent mine, which is why I can't advise you when to vent them. Depending how many you have, though, at 3 inches you should start to see some males emerging before long. IME by 3 to 4 inches males are starting to outgrow females, show more color, more male looking fins, as in longer, more pointed fins or fin extensions starting to appear. Really, by 4 inches you pretty much have young adults that may already begin pairing and attempting to spawn. Like a number of cichlids, from a decent sized group, you might get a few of what I call 'stealth males', those that take more time to show male, but I can usually spot these also by 4 inches.
At 3 inches you can still keep quite a few in a 75, plenty enough to assure a mix of males and females until you can visually pick out some males/females or some of them start pairing. If the idea is to sell some at this point and you're wanting to sell them as gender specific, my guess would be that venting may well be possible, maybe with some magnification, since other sexual characteristics are starting or will soon start to become evident.