The double crochet is normally abbreviated as dc in a pattern, so 3 dc means work 3 double crochet stitches. (In America, this stitch is called a single crochet.)
Put your hook through the foundation chain, or the stitch on the row below, or the ring, depending on which sort of pattern you are working. I’ve illustrated the stitch here working into a row of double crochet.
Wrap the yarn from the ball of yarn over the hook.
Now draw the yarn through the stitch – you will now have two loops on your crochet hook.
Wrap the yarn around the hook again.
Now draw the hook and yarn through both loops on your crochet hook.
You have now worked a double crochet stitch!