I sometimes wean at 3 weeks, if it's a small litter and they're fat. Go ahead and pull them into their own tub. Make the sipper low enough for them, mix the rat food with some kitten replacer milk to make it mushy morning and night, while also offering the blocks 24/7.
I think they'll be okay.
I hadn't thought about the kitten replacer milk! Ignore the goat milk comment lol.