Integrating dogs and cat

Has anyone here had experience with successfully integrating a mix of pets into their home?

The mix of our two dogs, and lone cat doesn't seem to be working very well.  To say its frustrating is a bit of an understatement.  The poor cat has had his home overtaken by two dogs and he is not very happy about this!

We currently have the upstairs closed off with baby gates so the cat still has his safe area.. The big negative is that he is too scared of one of the dogs to go outside via his catflap in the downstairs kitchen... Therefore we have had the window upstairs open for several months now! (Brrrrrrrrrrrrrrr!! )

Our smallest dog gets along great with the cat, the cat tolerates her too!  Problem is with the larger dog.
We introduce the animals daily to get them used to each other.. but there is still hissing from the cat, and shaking/barking from the dog.. 

Any advice would be greatly appreciated! (: Thanks!