Although not a shoaling species, platies do seem to like company of there own. There could also be an issue with the tetra being aggressive towards her, it will also be very stressed from being alone. The molly could also be showing aggression.
I also agree with Nick that the pH and hardness on very low for livebearers, both the molly and the platy, they need the minerals found in hard water.
If you are going to get another single platy, ensure that it is also a female. If it's a male, it will only make life worse for the female, as he will be constantly chasing her around, trying to mate.