Add 3 fields to invoice

-- url of the page with the problem -- : digitalguarddawg.com
-- HikaShop version -- : 2.2.2
-- Joomla version -- : 2.5.16
-- PHP version -- : 5.3.x

The client wants purchasers, during checkout at some point, to be able, or required, to add vehicle make, model and year.

How do I accomplish this please?

You can create custom fields of the table "order" via the menu Display>Custom fields for that (Business edition only).