First- have these horses diet and/or weight changed recently? (That is, are the fat ones fat from just grass/hay and is the thin one thin on the beet pulp/alfalfa/high fat pellets?)
Assuming this has been their diet for a while, I'd do anything you can to limit the fat horses' grazing. Grazing muzzles are a good start, but you may also want to eventually create a dry lot that they can be put on part of the time. Being obese is a bigger health risk for horses than being thin, so I'd be more concerned about them than the thin one based on your descriptions. I'd also look into upping their exercise if possible. Make sure the hay they're getting (when they're getting it at all) is the less rich stuff.
I second the addition of a ration balancer for all three horses. I took a look at Lonestar Feed's website
and they don't appear to offer one (although I was amused that their "Heavy Duty 14" feed is "for horses, mares, and colts" I figure that's kind of like the grocery store advertising their products as "food for humans, women, and boys"). Based on the feeds I saw there, I'd steer clear of their feed altogether.
If you can't find a feed store that carries at least Purina (their ration balancer is called Enrich 32), I'd look into a vitamin/mineral supplement, either from a local store or from an online store like SmartPak.
For the thin horse, I'd first double check when her teeth were last done (if it wasn't in the past year, have a vet do an evaluation to see if she needs to be floated), has any indications of ulcers, as well as her deworming status. Even if you've been deworming on a rotation schedule, consider getting a fecal egg count (FEC) done to confirm a low parasite load. Some worms don't show up reliably in the FEC, though, so a good full-spectrum dewormer is important. This is from Quest Plus, but gives you a good idea as to what different products cover:
Now, assuming those aren't issues
up the beet pulp and add some rice bran. What is the high fat pellet you're feeding (product name?)