Shouldn't three WEEK old bunnies still be with their mum? I'd not noticed that earlier, you mean, three MONTH old babies? If they're three weeks old, they should probably still be drinking milk. For the three month olds, if the salt is free choice, then they can eat it if they need it, I'd guess. Our sheep get sheep minerals in a separate dish and they seem to eat them when they want and leave them alone when they don't want. Wouldn't it be similar for bunnies?