At this point, you have three crumb-layer iced wedding cake tiers. You have the option of simply icing and decorating these tiers individually, and setting them up on a table decoratively, or using a tiered wedding-cake stand. This option means that you don't run any risk of collapsing, although you lose the drama of a three-tiered assembled wedding cake.
This is also the point that I suggest you stop, and leave the remainder of the decorations for assembling the morning of the wedding, at the wedding site if possible. Be sure to bring all of your supplies, including all the rest of the cream cheese icing with you.
The next step is to use plastic drinking straws to support the other tiers of the cake. Insert one straw into the center of the bottom tier, and make a mark with a pen about 1/8" above the icing. Remove this straw and use it as a guide to cut 4 additional straws of the same length.
Reinsert the center straw, and insert each of the remaining straws into the bottom tier of the wedding cake in regular intervals, about 2 inches in from the edge. Then place the bottom tier on the serving platter.
Place the middle tier of the wedding cake on top of the straws. It should thus provide a level surface for the middle tier to rest on. If it is uneven or unsteady in anyway, perhaps you need to either recut the straws, or add a little more frosting between the tiers.
Repeat the process, measuring and inserting five straws in the middle tier to support the top tier.