That really depends on what he and you like. My 3yo likes duplos, blocks, drawing tablet, trains, play kitchen, alphabet tablet, and matchbox/hotwheels car sets

That really depends on what he and you like. My 3yo likes duplos, blocks, drawing tablet, trains, play kitchen, alphabet tablet, and matchbox/hotwheels car sets. I don't think you need to worry about anything STEM specific, just find ways for him to practice dexterity and imagination. If you want to push for STEM stuff, he'll be much more capable around 5-6 years old (my 5yo loves Legos, puzzles, math, reading). In fact, your child is probably more likely to be interested in STEM stuff by watching you than you buying stuff for them. My older son got into Legos because my wife did, he likes video games because I play them (helped me build a RetroPie), and he helps me build/fix stuff around the house to spend time with me. At 3yo, he's probably dextrous enough to want to do cool stuff with you, but clumsy enough to get frustrated, so I think it's more important to build confidence and imagination than to push them in any given direction.