This version of a inc3 is a knitting technique to increase from one stitch to three stitches by knitting into the stitch below.
(IncR & incL version) Knit into top of stitch below first stitch on left needle then knit first stitch on left needle (see IncR) and then knit into the top of the stitch in the row below the stitch from the left needle just knitted (see incL). Increases to 3 stitches from 1 stitch.
See also: inc3 (K-fb version)