Welcome to quilting, Shayarene. The puzzle about half-square triangles is exactly as sewnoma said. But you know how geometry and trigonometry can give you some crazy answers with lots of numbers to the right of the decimal point? The actual measurement isn't exactly 2 7/8"... it's just the closest measure that all of us can cut from a ruler measurement. So, I'd vote for making those 3" squares and trimming them. I know, it sounds like a lot of work but, oh my.... what a difference it makes. Trimming has been the life saver on so many quilts I've made.