The controller in my group used a power which pushes the target 3 squares. He wanted to push him through the square of one of the PCs.
Is this possible? I thought that no-one could go through an enemy's square unless the power says otherwise.
You can probably move them through ally spaces, and probably not move them through enemy spaces. The rules are not explicit but strongly imply this.
You can move through an ally's space just fine in normal circumstances, as long as you don't stop there. I see no reason why forced movement shouldn't work the same way (though forced movement, just like regular movement indicates that you can't be forced into an occupied square).
The rules for occupied squares (RC 314) are more specific. You cannot enter (seemingly even by forced movement) an enemy's square unless they are helpless (or 2 size categories larger). So I would say this also applies to forced movement.
So you get the following from combining what we know.
You can force an enemy through it's ally's squares, as long as they don't stop there.
You can force an enemy into a prone ally's square.
And that's pretty much it.