I would plant clematis anytime, but now - in the spring is good because it'll have nice long time to get established.

Planting several in the same area's fine but remember that their roots are very deep so they'll need a good deep hole and they'll like a nice big area to let them run ..... so plant them about 2-3' apart and i'd say that should be fine.

That's my own opinion, others might disagree though.

I agree with you Louise 2 - 3 foot apart.

I would only add one more wee thing, just remember to bury them about a handspan deeper than they were in their pots, in case they get clematis wilt. Sometimes they'll succumb to it but if they are buried deeply enough they can re-sprout from below ground. It also helps to keep their roots cool which they love!! Welcome to Goy! Good luck! :0)