Three is when my son really began to be able to do involved pretend play -- with small cars, or Playmobil people, or animals, or whatever. So maybe something that would facilitate that? A parking garage if he's into cars, a dollhouse, or an airport, etc.
Also, we love MagnaTiles. We use them to build garages or a zoo or little houses for various people, animals. They're very quick and easy and satisfying to build with. Here are a couple different sets: http://learningforallages.com/MTiles.htm
Expensive, but have been worth every penny. I also agree with the outdoor vehicles. DS is into his three-wheeled scooter and his push bike.
Momma to 8 y.o. DS and 5 y.o. DD. Married to a Maker!
Woody Click, kind of like brio wood playsets crossed with legos. They come in different themes (fire, emergency, police, construction). The vehicles come apart and can be put together in creative ways. Ds turned a biplane into a rotary snowplow by putting the wings on the front (the plow), and the propellor on the bottom.
Ds also loved marble runs. We liked the translucent plastic ones although there was some breakage. The wood ones were frustrating because if you bumped them slightly, they would be knocked out of alignment and not work properly.