This technique among others is used to create decreases in crochet to eliminate one or more stitches; in this case you will be decreasing two stitches from your count. An external decrease is worked at the beginning or end of a row, at the side edge. Internal decreases, like half double crochet three together, are worked within the row. Generally you'll find decrease instructions in an individual pattern to suite the particular stitch being worked, but here are the basics on how to half double crochet three together.
[Yarn over, insert hook in next stitch, yarn over and pull up loop] 3 times, yarn over and draw through all loops on hook—2 stitches decreased.
You can also find instructions in Crochet Me's glossary for similar techniques including:
- Half double crochet two together (Hdc2tog)