P3tog, yon, P same 3 sts together again,
No, you don't turn the work but keep it all facing the regular way.
First you purl the 3 stitches together but don't slide them off the left hand needle. After that the yarn is in front, take it over the top of the right hand needle and on around the bottom of the right hand needle until it is in front again (that is your yon--another way of saying a yarn over- YO). Now you are in position to purl again. Purl the 3 stitches together again and slide the works off the left needle.
Where you had 3 stitches you again have 3 stitches one made by the p3tog, one made by the YO, and the 3rd made by the last p3tog. Stitch count stays constant.