I think it will be an option , you just inherit the sale.order module and add three new field in python then create its corresponding xml file and then put your desired fields what you want.

Ex. create a class A inherit Sale.order ... in class A add three new fields and then go for its xml creation then keep all the xml properties same as sale.order's just add these three new fields where you want to add after replacing the previous in sale order form.

But may be there many way to achieve your requirement so better decide how u want to ..