I have created a custom header menu called 'HM-1'

And i have created a Menu called 'Menu 1'

Inside 'Menu 1' i have 3 fields created, called field_1 fileds_2 filed_3.

I created a 'User' and i have two users now

1. Admin
2. User

My question is 'How to make one field(field_3) 'User' and 'Admin' editable in Openerp-7'

Remaining fields in 'User' should be readonly only field_3 should be editable.

How to do this?