My 3yr old has been getting up to 104 fevers, rashes, and hives, what's going on?

Hard to tell: Proper assessment will require some history, an exam & possibly lab work. This could be as simple as a transient virus or a more worrisome auto-immune disorder. I would have this evaluated by the child's physician when you can arrange it. If the kid is taking fluids well & playful at least part of the time it can wait until office hours. If the kid is not, start with an urgent care center.

Hives: Please give one dose of children’s Benadryl and see urgent care right away. Most likely, there is an urticaria response to a viral infection. Other triggers of an allergic reaction include certain foods. Watch for signs of respiratory distress and drooling.