Odoo 13 - sale.order.line

I have fields in a tree view:

<field name="field1" optional="hide"/>

<field name="field2" optional="hide"/>

<field name="field3" optional="hide"/>

<field name="select" optional="show"/>

When I make a selection in the field "select", I want set field1, field2, & field3 to "show" using @api.onchange('select') in the model.

How do I do that in the model?