There are many ways to mentally stimulate a 3 year old.
Here are a few ideas:
Play games that challenge their problem-solving skills, such as puzzles or board games.
Encourage them to use their imagination through creative activities like drawing, painting, or pretending.
Introduce them to new experiences, such as taking them to the park or a museum, or trying a new activity like cooking or dancing.
Read books together and discuss the stories and characters.
Teach them new words and concepts through everyday activities, such as counting objects or identifying colors.
Overall, the key is to provide a variety of engaging and stimulating activities that will help your child develop their cognitive, emotional, and social skills.