When you k1,p1,k1 into one stitch you do this:
Knit into the stitch as normal, but don't slide it off. Bring the yarn between the needles to the front and purl into the same stitch but don't slide it off yet. Bring the yarn back between the needles and knit into the same stitch again and then slide it off. You'll have three stitches where there was one.
In case you were asking about p3tog, also, just slip the needle into all three of the next stitches, wrap once and pull the yarn through the three stitches as if they were one.