I am doing another 3D building but the last one I couldn't get the corners sharp. How do you do it? Would modeling chocolate work better than fondant for decorating the cake?

This cake is 2x the size of the last one I did and I really want it to come out better.

I typicaly roll and cut the fondant into the wall shapes a day or so before I assemble the cake. Then apply the panels to the sides of the iced cake using a little vanilla to "glue" the corners together. This requires a lot of planning and very precise cutting, but you can achieve really great corners.I have a few cakes in my gallary that were done this way, baseball stadium, church building, the Eiffel Tower and part of the locomotive. You can also cut out nice square window openings too, and place another piece of fondant on the inside that is a little larger than the opening.
It does make the fondant a bit more difficult to cut when serving but that's the trade off for the sharp edges.

I hope this helps.
Rick

Thank you... that is going to take up a lot of table space with all the pieces. Will it soften up some once put back on the cake? What kind of fondant do you use?

The inside of the fondant that is in contact with the butter cream should stay softer. I normally use Wilton brand for somthing like this that needs to be a little more firm. As for table space, you can have a sheet of plywood cut into 6 16x45 inch pieces and 12 1.5x16 inch strips. I place parchment paper over the first board with as many fondant parts as you can, leaving room at each end for the 1.5x16 strips, then stack more layers as needed.