I am making a display cake for a wedding this weekend.  The guests will be eating my cupcakes so the bride and groom just wanted a cake for display with a top tier to cut.  The botttom tiers will be 12 and 9 inch covered in fondant and the top will be real.  I'm just wondering how and when to assemble.

I originally planned to assemble the whole thing there- by just securing the tiers with a little buttercream between the tiers.  But now I'm wondering if it would be stable enough to assemble the bottom two tiers before delivery and add the top tier at the reception.  Since they are so light I'm just worried they will tip over.  Also, it's ok to place a real cake on top of two tiers of Styrofoam right?  No weight/stability issues to worry about?
 
Thanks!

I am assuming that your top tier is a 6" then. I dont think there should be a problem at all. Dont secure it with buttercream. I would say use royal icing instead. for the  two fake tiers just send in four skewers and it wouldnt budge.  at times i wet a piece of fondant and place it in between tiers and it has never moved. if you are still worried abt the top tier moving send a dowel /skewer through from first to second tier, thats only if there is a topper on the 6" which will cover the little hole thats on top.HTH

If you don't have a topper for the top tier you can cut a small hole in the centre of the cake board that will hold the top tier real cake. You can cut a centre dowel that is slightly shorter than the height of the three tiers. Sharpen one end. Put it through the bottom two tiers pounding gently with a mallet. Looking at the cakeboard underneath your top tier, feed the dowel through the hole in the cake board.

 Good idea

suzied, I have had to do this a number of times and it works well. Just have to remember to make the hole in the board.

Also, I have covered a few two tier dummy cakes with fondant and if you are worried about them sticking together, you can take a utility knife and remove a bit of the fondant from the tops of the two dummy tiers and put a bit of glue to secure the tiers to the bottom of the boards of the cakes above.

I usually secure mine with melted chocolate

Thanks for the tips everyone!  Here's the cake.  I took the bottom two tiers assembled, "glued" with melted chocolate.  Then assembled the top tier at the reception hall just using buttercream.  

http://www.cakecentral.com/gallery/i/3345695/navy-gold-and-black-three-tiered-cake

Lovely finished cake.  Well done.