The p3so is a decrease worked as follows: pass third stitch on left needle over first two stitches and off left needle.
In the case of the Mendia Hat and Mendia Cowl patterns, the first two stitches on the left needle are then worked k1tbl, yo, k1tbl. The combination of the p3so and following stitches create the distinctive eyelet pattern and will result in 3 stitches on the right needle. as the yo balances the decreased stitch.

Step 1: Using the right needle, lift the third stitch on the left needle over the first two stitches.

Step 2: Leave this stitch off the needle, 1 stitch is reduced.

Step 3: The decreased stitch looks like this, wrapped around the base of the first two stitches on the left needle.

Step 4: Follow the pattern instructions for the two stitches left on the needle, in the case of the Mendia Hat and Mendia Cowl patterns, work these two stitches as k1tbl, yo, k1tbl.

Step 5: This is how the p3so k1tbl, yo, k1tbl instruction looks after completion. (As instructed in the Mendia Hat and Mendia Cowl patterns)