If you brood in the tub I'd close the drain spout and tape some heavy duty plastic over that. You do not want to plug up the drain and get a plumber's bill to fix that. You need to be able to clean really well when they are gone. Protect that drain.
I'd be tempted to spread heavy plastic on the floor if they can roam at all to keep their poop from staining it. Close the toilet lid and keep it closed. Before too long they will be able to fly. You don't want them to fly into the toilet and drown.
As I mentioned earlier, if they are confined to a crate or box, spread cardboard under it to protect the floor. Or use heavy plastic. For only three chicks a bathroom that you don't use isn't too bad a spot. But still protect it and look around for hazards they can get into. They are remarkably good at getting into trouble.
I'd be tempted to spread heavy plastic on the floor if they can roam at all to keep their poop from staining it. Close the toilet lid and keep it closed. Before too long they will be able to fly. You don't want them to fly into the toilet and drown.
As I mentioned earlier, if they are confined to a crate or box, spread cardboard under it to protect the floor. Or use heavy plastic. For only three chicks a bathroom that you don't use isn't too bad a spot. But still protect it and look around for hazards they can get into. They are remarkably good at getting into trouble.