When you start up Trine 2 on PC (and Mac), we are making sure that all of the possible controllers out there are configured correctly.
So far we have bought, tested and binded button configs for about 15 different pads and expect the number to grow to about 20, thus hopefully we can cover about 90% of the options out there.
Keyboard and mouse support will still be available and better than ever, but if you want to play 3 player local co-op, I think using additional controllers might be the way to go. That is unless you and your friends really really like the key/mouse combo and have a lot of table top space available. 🙂