Ok I'm in Australia so the feeds available here are going to be different BUT I've brought home a 27 yr old mare in an emaciated state and got weight on her in six weeks. This was my feeding regimen with her (I'm not saying you need to do this but this will give you an idea of how easy it is to get weight on a poor horse):
Morning - one or two biscuits of hay (we call a flake a biscuit) and hay varieties I used were rye grass, oaten hay and then I introduced Lucerne hay PLUS two dippers of Maxi Soy prepared feed Maxisoy+ Australia
Mid-morning - a biscuit of hay
Lunch - a biscuit of hay + one scoop of Gumnuts prepared feed for older horses
Mitavite - Scientifically Balanced Feeds
Late Afternoon - a biscuit of hay
Night feed - 2 bisuits of hay + 2 dippers of Maxi Soy.
I gradually built up the quantities, starting off with simply rye grass hay and 1/4 of the quantities of prepared feed. But in the six weeks we had her before she crossed the bridge, she went from emaciated to nearly normal body weight. She died of old age, but had she not been starved by her previous owners, she may have lived a much longer life. We know she had been a brood mare and had foaled last season.
So my suggestions are: introduce a good quality prepared and complete feed; change hay suppliers or at least find out what composition your current hay supply has (there is so much variation in hay quality which affects its nutritional make-up); separate the horses and weight each animal's feed and ensure each animal only eats its own feed; accurately weigh each horse now to determine its actual volume requirements (weight of feed per day each horse needs to gain weight, not maintain); take photos of each animal now so you can monitor their progress and; ensure each horse has a thorough examination by a trusted vet as there are so many problems that are indicated by poorer body condition. Best of luck...your horses can't speak for themselves so it's up to you to speak up for them and maybe it would be best to re-home one so they can all be cared for to the level they each require.