Why do Dogs and Cats fight...or is that just a myth?

i have had 2 cats for seneral years now and we have adopted them from shelter where i volunteer and now we have taken another animal she is a beagle daushund mix froom there // the cats seem to be very territorial especially one how do i solve this problem short of getting rid of all of them which I do not have the heart to due since I saved them in the first place...