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. 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.
... although gamers still will not be able to play against someone using a different type of console.