When were they weaned off the bottle? At 3 months, they are just about the right age to be able to handle a straight hay and grain diet. They should of been on a predominant milk diet for the first 6 weeks and the last 6 weeks been added more and more feed stuff while cutting back on the milk. Before that age, they do not have a developed rumen to break down and utilize long stem fiber. My guess is they were taken off of milk too early and have been basically starving themselves while they wait for their digestive tract to develop enough to process what they are eating.
Stop with the deworming. That is not your problem. Lack of calories plan and simple is the problem. I would restrict them to a small pen and shelter. A large pasture might look like the right diet but they are hiking around buring what precious calories they are ingesting walking not growing. Get them on a diet that is 50/50 grass alfalfa. Get them on a calf starter. Cracked corn has decent calories but it's a low protein and unbalanced feed that isn't going to help right now. Give them just a pound to start. Let them clean it up. Give them another pound for lunch. Another for dinner. They need to double their weight then start letting them out to graze. Build a creep feeding pen that they can get into but the horses can not. Keep some alfalfa for them to eat at will.
I would pick up a tube of calf saver too. It's a vitamin, mineral and probiotic paste that often stimulates the appetite.
