Salut mon gars, welcome ! :)
The side screens of your TH display should be oriented slightly towards you, but honestly, monitor orientation will not really help if you experience motion sickness in 3D games...
Try Bezel Management, I know some TripleHeaders don't care about it, but to me 3D doesn't feel right if the transition between each panel is not as smooth and natural as it can be.
You might also find this video interesting :
The Truth About Edge Distortion
TripleHead movies don't exist afaik, so you obviously can't watch a video in 5040x1050
unstretched.
Yui is correct, the GXM/PowerDesk app lets you setup a list of single-, dual- and triple-screen resolutions that you can use at will, so you can for instance switch to 1680x1050 on the center panel then switch back to 5040 at anytime.