The content does not get cropped at all, it gets stretched to fill the output. On a 2x1 collage its not terrible, but going wider than that you will definately be able to tell the content has been stretched.
I've never used DL3's before, but from my experience with DL1's and Catalyst, I believe I can adjust the X and Y scaling and crop some of the top/bottom if I want to?
yes you can adjust the scaling of each image on the graphic layer under Beam. There is Image X, Y and Z Scale. If you want to crop the top and bottom you could use another graphic layer and make your own matte. I don't have a DL3 in front of me but I bet you could use an effect(located in under Color) such as horz slats to crop the images.
TransWipes is a great effect to use for that purpose. Ben Richards used this on the latest Rob Thomas tour for some special requirements of his projection screen.