A 3yo doesn't need a "stem" toy. They just need access to safe objects, heavy cardboard or plush books with simple colorful pictures, etc.

In short, they just need access to things they can observe and manipulate.

While a parent can to some varying degree influence the attention and interest of the child, there is plenty of doubt as to whether this will result in a better outcome long term or not.

As other people have said, one of the best things you can give your child is your time (and undivided attention).