With ours we did use an Xpen to keep them from the cat and the kitty would come up and "investigate" the new furball.
The unfortunate part is that this excited the little guy so much that it scared the cat!
It took some time, but typically the cat can "get" to places that the dogs can't.
Now they are all really good friends and one cat loves to instigate something with them at all times because she wants to play.