3yr old has occasional leg/knee pain usually in one leg going on and off for 2yrs mostly in the evening, now says finger hurts sometimes. no swelling?

Pain: It is hard to pin point the reason unless there is more extensive hisotry, including history of fevers, rashes, limping, swelling, morning stifness, mouth ulcers. It also should include family history, then examination to see if there is any heart murmurs, rashes, or swelling.

Children certainly: can have intermittent aches and pains in extremities and joints, which can be for no apparent reason.If he hasn't been evaluated by his pediatrician,he should be;the doctor may want to so some screening lab tests.You could also take him to a pediatric joint specialist(rheumatologist)or orthopedist.Also,make sure he's not imitating an adult who is saying this, especially if his workup is normal.