Their feathers are water resistant. When they get into the roost they will puff out their feathers and shake off. As long as they aren't stuck in a shower they will be okay.
If you have bigger than standard, anything higher than 3 feet is too high. I'd go 24 inches. This would give you options in how deep you want the straw/sand/wood chips,... to be, and not be too high. If it is too high the hens can damage the egg.
If you only have three chickens I don't see why you would need two roosting bars. They will will want to roost side by side. Even if one doesn't want to cuddle up one roost would give her a few inches to seperate.