Auto Generate field based on other fields

I was wondering if i can create a field that will auto generate using what i picked in the form for example.

I am creating a form for clothing inventory and I have the following fields

Brand - Nike
Type of clothing - Shirt
Size - L

I was wondering if i can generate a autonumber based on what i pick./

So it wuld generate NIKSL003

This number would not repeat

Any Ideas??