Draw it out on graph paper. Three inch strips will give you 2-1/2" finished. Since 12-1/2" is not evenly divisible by 2.5, you will have to fudge the last rows. Otherwise, you can leave them full width, and have a different size quilt. Kinda-sorta sounds like you haven't thought your project out.
Why are you choosing 3"?
A great beginner log cabin book is Quilt in a Day Log Cabin by Eleanor Burns.