If they were somewhat starved before they came to you, you need to be careful about giving them too rich of food too quickly, as they aren't going to be used to it. I would provide unlimited grass hay and watch to make sure they are eating it well. If they seem to be doing alright on the alfalfa, I would continue to be giving them some of that, but with alfalfa you do have to be careful because it's high in calcium and that can cause bladder issues for some rabbits. I would wait a bit on the oats until you have their diet somewhat stabilized, then maybe try gradually adding some in, and keep a close eye out for the oats causing any mushy poop. If they are eating the grass hay and alfalfa well, I would probably wait a week to make sure their digestive system is doing well on the hay, then I would gradually start adding in a good quality pellet, starting with a small amount and gradually increasing, always keeping an eye on their poop to make sure it stays good. Then after several weeks of doing well on pellets and hay, gradually start adding in veggies, one at a time starting with small amounts. I would also give them a salt lick while they are only getting hay, as they won't be getting the needed sodium from pellets.
Also keep a close eye on water consumption and whether or not the rabbits seem to be drinking too much or too little.
Once they are on a stable diet their weight should gradually increase unless there is an underlying health issue causing the weight loss. If you have a scale it would be a good idea to track their weight every day if you can, or at least twice a week. If they don't appear to be gaining, it would be a good idea to get them checked at the vet if you haven't already. They could have parasites causing the weight loss.
