A sc3tog is worked when 2 stitches need to be decreased. When working a trim, as in the Star Wash Cloth, a sc3tog is worked at the bottom of a V to draw in the two edges. Here's how to do it.
Single crochet to where the sc3tog will be (Photo 1).
Insert hook into the first stitch used in the sc3tog (Photo 2).
Wrap the yarn over the hook and draw up a loop (Photo 3).
Insert hook into the next stitch used in the sc3tog (Photo 4).
Wrap the yarn over the hook and draw up a loop (Photo 5).
Insert hook into the last stitch used in the sc3tog (Photo 6).
Wrap the yarn over the hook and draw up a loop (Photo 7).
Wrap yarn over hook and draw it through all 4 loops on the hook (Photo 8).