How to show my 3 years old to be gentle with the material?

Modelling how to handle materials is the first recommendation, time and time again. Ask another child to show the 3-year-old how to handle the material.  Additionally, immediately step in when materials are being mishandled and stop the activity. Montessori was very clear that stopping inappropriate activity does not harm a child; allowing a child to continue doing things that are not to their benefit is more harmful.

Child's hand on Montessori material