Is my rat ill, or am I hysterical? What do these symptoms mean?

Three years old is quite old for a rat, most rats rarely make it too far past two. It's possible that he is slowing down due to old age, however I would still try to get him to a vet just in case it is something that can be treated. Even a large animal vet should be able to examine him and give him some fluids or notice any heart disease or other symptoms of something else you may have missed. I hope he gets better soon, good luck!