Complicated question. It depends on the kind of fish. Certain fishes are more likely to impart their flavors into other meats, such as salmon and tuna. It's possible to do, but it's not recommended. Many people maintain a separate smoker just for fish, or they thoroughly clean their smoker after doing fish and before doing other types of meat.
Also, fish ideally should be smoked at 180, whereas beef and I think pork should be smoked at 200-225. You can smoke beef at a lower temp, but you don't want to smoke fish at a higher temp.
Lastly, depending on how sophisticated you are with smoking, you may want to use different types of wood for fish vs beef.