3-5-3 Random, Unique Alphanumeric Code Generation

My end goal is to have an 11 digit code. We want to have a 5-digit random but unique code that will be attributed as a promo code. We then want to have 3-digit random but non-unique codes before and after the 5-digit code. We want to keep the order of number, letter, number, letter, etc.

I figured the easiest was to do this would be to have three columns with the five digit code, so we know what that is, and then combine all three columns into a fourth column.

I've added an example spreadsheet to help show the idea. I'm just trying to figure out what the easiest way to process this would be.

Thanks!