Does MW3 support cross-platform multiplayer?
Lets say my friend has a ps3 and I have an xbox360, will we be able to play modern warfare 3against each other in multiplayer?
No as Sony uses PSN and xbox uses microsoft xbox live they are not the same the only way to play together online would be on the same system PS3 to PS3 or Xbox to xbox Sony would never share the stuff needed with microsoft vice versa.
No. Following the trend of pretty much any current console game out there, there is not cross-platform multiplayer.
However, you can compare stats against your friend using the Elite service. In this article about the Elite service, is the source for the answer against cross platform play.