You can roll a tent in a way that it is a pretty waterproof package. The floors are quite waterproof so with that on the outside I have never felt the need to protect the packed tent from the rain. My tent is always packed on top of the rack and never in a pannier. I never bother to cover it or put in in anything other than the sack that came with it (I'm not even sure if that is waterproof, but I think not). It does not seem to ever get any wetter while rolled up even in the rain all day.
My sleeping bag always gets stowed in a waterproof pannier or in a plastic bag in a non waterproof pannier.
My sleeping pad travels usually travels in it's non waterproof stuff sack on the rack too, but gets a plastic bag over it when it rains. I have also carried it in a pannier though.
It isn't the end of the world if my sleeping pad gets wet. It can be toweled off pretty quickly and well, so of the three items the only one that is a huge deal about keeping dry is the sleeping bag.