My cat has fever, swollen belly, and red gums. Could it be FIP?

I understand you are concerned about FIP. The reality is, if you don't have money for a veterinary visit (and in many cases even if you did), FIP is typically fatal and untreatable. The best thing you can do is separate your sick cat from your healthy one and see how things transpire over the course of the next few days. Typically, cats with FIP, regardless of the form, will be inappetant and not wanting to eat anything at all. The worst form of it will cause massive fluid buiid up in the chest / abdomen and result in difficulty breathing. The other form typically produce clinical signs that are both ophthalmic and/or neurologic, plus several other non specific clinical signs (a couple of which include what you are descrbing). Overall, FIP would not be near the top of my list. I would definitely make sure all three cats have been tested for leukemia and AIDS however in order to rule both of these life-threatening conditions. While not as rapidly fatal as FIP, both conditions can carry poor long term prognosis. It sounds like the one cat with a fever and swollen belly might benefit from pain management and a recheck for the presence of worms. While dietary indiscretion could be an issue, it is not likely to cause a fever unless an infectious organism was present in the food. Hope this is more helpful than my previous response.