My hunch is that, because you have three males, the males are territorial towards one another and are avoiding coming out in the open for fear of getting into fights. Although swords are peaceful towards unrelated species, males are very possessive towards females, so most hobbyists keep just one male per tank, with a couple of females so that the male spreads his attention among them instead of constantly picking on just one.
Assuming that you are going to keep all of the fish, you could try rearranging the plants, so that the plants make a sort of divider down the middle of the tank. The more you can give each male a territory from which he cannot see the other males, the better. Try to give every fish a safe place they can retreat to, which will let them feel more free to come out in the open. You might also drop the temperature to around 76-78.
Some people swear by salt for swordtails, and some swear that it's completely unnecessary. It's certainly less useful than for hardwater livebearers, so you could go either way with it. But salt tends to be bad for live plants.