What does dc3tog mean in crochet pattern?

This means double crochet 3 together. It is a decrease over 3 stitches.

yarn over (yo), insert hook in stitch, yo and pull through, yo and draw through 2 loops (2 loops on hook)

yo, insert hook in next st, yo and pull through, yo, draw through 2 loops (3 loops on hook)

yo, insert hook in next st,yo and pull through, yo and draw through 2 loops (4 loops on hook)

yo and draw through all 4 loops.