Trail a bike, or the gator bar (we used both). 3 years is a bit trickier, until they can reliably stay on the trail a bike.
At age 4, we also started walking (fast) while DS used a push scooter instead of biking, and DD (age 6) would ride her bike (she would wait up, or ride circles, or whatever). 2 miles one way was about as far as we ever needed to go that year, then the Trail a bike started up.