It's in the beginning notes, not in the line itself. Here's the pattern link - http://www.debbiemacomber.com/index....nnp&pageID=195
and in the Beginning to Lace Knitting intro she explains it:
"The 3-to-1 decrease combines these steps: (1) insert your right needle into the next stitch on the left needle as if to knit that stitch, but instead just slip that stitch onto your right needle, (2) knit the next two stitches together, and then (3) lift up the slipped stitch and pass it over the newly made stitch."
So it's a slip 1, k2 together, psso decrease.