Are you asking how to do something like "A"
If "A" is what you are trying to do, the squares need to be a "couple of threads" larger than 2.25 inches (cut size)
The actual size is the ((square root of 3.125) plus 1/2))
(1.768 + 0.500 = 2.268)
For the triangles on the sides - I would cut squares 2.25 inches (which is a bit over sized) - cut them in half diagonally - sew them on the square - and then trim the square in a square unit to 3 inches - which should end up being 2.5 inches "finished" size
(I think the math is correct - feel free to "do it right" if it's incorrect.)