Got three in a big two level hutch out back.
Having Three does mean they curl up together which helps them in cold weather.
I put clear acrylic sheet over parts of the mesh to keep the rain and wind off them in winter and also foam camping mat and a tarp over the top half if its below zero. This makes a big difference, their water does not freeze with this until about -5.
A very good idea is to line the whole of the sleeping area in 25mm (or more) polystyrene foam with plywood over it. With even one rabbit in it it heats up very well. We had down to -10 last winter with no problems.
If the rabbits ears are cold to the touch, the rabbit is cold ! But resist the temptation to bring it into a nice warm house at this point as the tempreture variation is too much for them to handle. Just give them some food and plenty of bedding hay.