Now I have records like fields 1 & 2,

and want record auto fill like field3

but meet some difficulty

Here is my try

    @api.depends("field1", "field2")
    def _compute_field3(self):
        for rec in self:            
            if self.count(rec.field2) > 1:
                rec.field3 = rec.field2 + "-" + #the subscript
                # Maybe somthing like this
            else:
                rec.field3 = rec.field2