Working into a base of an even number of stitches:
*1 dc in each of next 2 stitches, 3 chain, slip stitch in left-hand side of last dc worked. Repeat from *.
To make the slip stitch, you need to insert the hook into the 2 vertical strands at the side of the last dc worked.
I have used UK crochet terms. American crocheters, just read sc instead of dc.