This. Three years old is a good time to build an intuitive understanding of the simple machines (lever, wheel and axle, pulley, inclined plane, wedge, screw). Hands-on play with this kind of thing will set up a solid foundation for more complicated STEM when they're older, and the kid won't even know they're being educated.

Mannnnn remember getting a new large appliance and anxiously waiting for the box to be empty so you could go climb into it and turn it into a spaceship or submarine or something else?