When you create a custom product you can create a 3rd Dropdown Option on the Product page.
There is a limit of 100 total variants, so depending on the product, the amount of variants you can create is limited.
The 3rd Dropdown Option can be several different things, for examples....
These are just some ideas.
Whatever you create as option 3 on your product page, you will need to upload a matching printfile psd design file for that variant, so we can fulfill the order when customer orders.