Well, I think sometimes best friends is too much to expect, but I'm sure you can get all your pets to at least tolerate each other. Does the cat behave in a threatening way or something towards the dog that's scared?
Anyway, with dogs (and cats too of course) I think the best way to go is that when the cat is around, the dog gets wonderful things! Whatever that is to your dog. If the dog likes treats, start giving her those when the cat comes around- downstairs at first, and as she gets used to it, as the cat comes closer etc. Just deal them out liberally. And do that each time the cat comes around, slowly the dog should start associating the cat with those wonderful treats! If it's not treats, a favourite toy, or some petting, or whatever makes that dog happy. Calming signals may work too, dogs use those on other dogs when they want to calm the other one down, things like yawning, or turning their head away, licking their lips etc. You could try those, yawn and lick your lips, and see what happens. Dogs are very good at looking at their owners for how to react, and if you seem anxious or nervous, or cuddle the dog by speaking in a "comfort" voice usually makes it worse, if you appear confident and show there's nothing to be scared of, the dog will most likely start acting according to that too eventually.
You didn't say how long you've had the cat, but sometimes it can take a while for pets to get used to each other. Dogs and cats don't always understand each other terribly well, so it's a learning process for them too. Your cat might be a little scared of the other golden too, hence the hissing. Just make sure the cat has places to go where the dogs can't get to, and of course making sure the cat doesn't bother the dogs either when they want to be left alone.
I have two dogs and two cats, and it's always taken a few weeks for everyone to settle down, and much longer than that before they've become friends or become totally relaxed in this new "pack" of theirs. I just always try to make sure everyone has place they can go to when they don't want to be bothered, that no one picks on the others, and let them get used to the new addition in their own pace.