When my son was three, it was tractors. We lived in a suburban neighborhood, not near any farms, and we didn't know anyone who owned a tractor, but tractors were his entire life for some unfathomable reason. We had bought him some toy trucks and cars, and one happened to be a tractor, and he just latched onto that thing. We had to buy more toy tractors, and before he could go to sleep he had to separate the tractors from the other toys, and line them up JUST SO at the foot of his bed. He knew which direction they needed to face and in what order they needed to be. If we saw a commercial for John Deere on tv, oh my, the world had to stop so we could all marvel at the miracle of tractors in hushed silence. If a kid's tv show had a tractor in it, he would stare in frozen rapture. He would carry his tractors everywhere and hug them.
That boy eventually moved on, to sticks, then to Peter Pan (he stopped calling me mommy and addressed me as Mr. Smee for a few weeks, and I had to call him Captain Hook), then to fish. Then he named his feet. The right foot was Ben and the left foot was Hedgehog. If I wanted him to come to the table, I had to call Ben and Hedgehog too, because of course your feet have to go where you go! He would talk to Ben and Hedgehog and ask them where they felt like walking, or if they were tired and needed to rest.
He's now a grown, responsible, very smart adult, with a college degree in a technically demanding field, travels the country for his job, and is happy and healthy. No tractors are involved. He's an audio engineer and monitor systems engineer with a sound production studio, and he loves what he does.
Sometimes I wonder if his natural ability to focus so intently, and his creative mind, and his imagination, and his decisiveness helped him become the engineer that he is today. No one in our family had ever heard of audio engineering when he told us that was what he was going to become. And he's really really good at it.
Enjoy your son!