You have your single long piece of fabric folded in half and have sewn a seam down each of the long sides; we'll call these the side seams.

On the one short side that is "open", you want to sew in from each edge an amount that will be slightly larger than the width of the "box" (the 3" perpendicular seam that you mentioned at the front and back corners); we'll call this the end seam. The unsewn part of the seam on this edge will allow you to tuck the pillow form into the pillow after you have boxed the edges.

Now to box an edge: Put your finger in the "point" where a side seam meets an end seam. Position so the two seams lay atop one another. You'll have a triangle. Mark in from the point of the triangle until you have a line forming the base of the triangle that is 3" long. The overlapped seams should be centered in this base line, so you actually are sewing the base of the triangle 1 1/2" to the left of the centered seams and 1 1/2" to the right of the centered seams.

Now when you turn this area right side out, the large triangle you have created will be inside the pillow and you'll see a 3" seam perpendicular to the front and back corners.

Do this on each corner. On the folded end "seam", there is actually no seam to guide you, so mark the fold with a pencil or pins, so you have a guide for lining it up with the side seam to sew the triangle.

The link below pertains to making a purse. However, in steps 7 they use the triangle technique to make the purse bottom. The instructions for the purse refer to a lining; you can ignore this. Look at step 7 and the pictures provided.

June

Here is a link that might be useful: Boxing a corner