Question 64885
This is a variation on an old problem but it does have a solution that does not require the man to put the mouse in his pocket (not a bad idea, though).
The basic idea is to never leave the dog and cat alone together or the cat and the mouse alone together. 
We have to make an assumption that the dog would not eat the mouse if left alone together.
1) The man takes the cat across and leaves it on the other side and he returns
2) The man takes the dog across and leaves it but he picks up the cat and returns with it.
3) The man picks up the mouse but leaves the cat behind and takes the mouse across and leaves it with the mouse-friendly dog and he returns.
4) He gets the cat and takes it across, and he's done.