The question is: how big a piece do I need to cut to have a 1/4" seam allowance and fill in the space in the corner from cutting off those 3" x 3" x 4.25" triangles? Trust me, if those spaces are not 3" x 3" x 4.25" geometry professors will beat down your door because you just disproved Pythagoras' famous theorem.
Here is what you can do to do the least work. If you have any fabric left that matches your snowball bodies, cut it into 4" squares (the actual theoretical size is 3 7/8"' but I don't cut that accurately). Starch it before cutting. Cut each square along the diagonal, giving you two equal sized triangles.
Carefully, because you have bias edges, match the long edge of this triangle with the long side of the space in the corners. Part of the triangle will be longer; that is the way it should be. They look little ears. The raw edges should meet 1/4" in from the cut edge, where your seam line will be. Sew the two pieces together using a 1/4 seam. Press the triangle piece into the corner. Press, don't iron. Trim off the little ears. You may need to trim off a little bit on the outside edges of the triangle, but it will only be about 1/16".
If you need to buy more fabric, you can cut about 10 squares or 20 triangles from each 4" of 40" wide fabric.