ACan someone tell me how to stack a three tier pillow cake without tipping over, should I assemble it before or at the venue[SIZE=3][/SIZE]

Anything over two tiers(for me) should always be assembled at the venue. 

I would stack at the venue and remember, under each cake you need a cake board with dowels under it so it can support each cake. It will probably need a center dowel or two to be sure the cakes stay in place. Post a photo!

if you stack on site you need each cake sitting securely on a larger board (that will be discarded) just so that you can successfully transport and pick up each tier without marring the edges --

i would deliver this stacked myself