I have to do a 3 tiered wedding cake this weekend. The 2 bottom tiers are 'fake' and the top is 'real'. The wedding is 2 hours away. Should I put it together before delivery or after? and what support system should I use?

I would pre-assemble it, if 2 of the 3 are fake anyway. No, I would not use a support system just for one layer (tier) of cake. The fake cakes will in effect be glued together with the buttercream icing and wont' move. I'd put some BC under the top tier, to attach it to the middle tier, and it will hold fine.

If you're really really nervous about it, pre-assemble the fake tiers, then just plop the top tier on when you get there.