To encourage the appropriate three-point grip of a writing utensil, try telling a story about a duck/alligator that eats the writing utensil.  Here is an example.

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I have this duck on my hand you see.  He eats pencils, pens, markers and anything else in our writing tree.

He helps you hold  this writing utensil,

So that you can write things like your name with a pencil.