I bought my friend’s three-year-old son Gears! Gears! Gears! as a holiday present and he absolutely loved it. Within moments of unwrapping it, he was building machines. The surprising part was just how much and his mom and I loved it as well.  If you are looking for for a STEM toy for a three-year-old who you want to encourage to be creative and build things, I recommend Gears! Gears! Gears!.