If you are trying to match up 3 2.5 inch strips (3 Rail Fence) with a Snowball, I would sew 2.5 inch squares facedown onto the corner of the 6.5 inch square you're using for your snowball. Then clip the corners and flip up the remaining triangle.
Here's a link that shows Snowball & 9 Patch which is sort of what you're doing only you're using Rail Fence instead of the 9 Patch.
http://www.quilterscache.com/S/Snowb...atchBlock.html
Here's one that just shows the Snowball:
http://www.quilterscache.com/S/SnowballBlock.html