Hi Chris,
On the inside of most Duncan Hines packages, there are instructions for large tier cakes. I have on many occasion used Duncan Hines to make wedding cakes and have had great success. My favorite icing recipe is on the back of the Wilton Meringue powder container (may be purchased at any cake decorating supply store or even Walmart), the buttercream recipe. I would use wax paper to line the pans as there is nothing more aggravating when a cake sticks even though you greased and floured the pan! Good Luck!