Help with phentex slippers!

Hello all!
I’m creating classic phentex slippers for the 1st time (I am truly a novice knitter!). It calls to cast on 40 st. At the 2nd row, I am confused by the following within the row:
(K1. P1. K1) all in next st. P3tog.
I repeat it, having this twice at the beginning of the row and supposed to have it twice at the end of the row.
As I continue to the end, I haven’t enough stitches left to finish the second to let alone the rest of the stitches that are recommended.
To be clear, I am actually K1 P1 K1 and Purling 3 together in that part of the pattern.
What am I doing wrong??
Thanks to anyone that can help!

When it says (K1, P1, K1) all in the next stitch, it means that you will knit the stitch and before you slip the original stitch off the left needle you will bring the yarn to the front and purl the same stitch. Still, before slipping the original stitch off the left needle you will return the yarn to the back and knit the same stitch once again. You have made a total of 3 stitches onto the right needle and the original stitch from the left needle can now be slipped off. The next stitch, P3tog, means to purl the next 3 stitches on the left needle together, as though they were one.

KnitQueen- you’re amazing! Thanks so much, that makes total sense to me!
Thanks from your novice counterpart in East Canada!