purl 3 stitches together
P3tog is a 2 stitch decrease for the purl side of your work. It means purl 3 together, so you’re creating 1 stitch out of 3 live stitches.
To work this stitch start off like you would normally for a purl stitch except your right needle is going to go through the first 3 stitches on your left needle. You’ll now purl as normal, bringing the yarn around and picking up the new stitch whilst slipping the old stitches off the left needle.