P3tog (purl three together) is a double decrease that presents as right leaning on the right side of the work. It can be a bit fiddly getting your needle into three stitches at once; needles with a pointy tip can be helpful, as can using a yarn made from fiber that has elasticity (wool). Scooting the stitches near the tip of the needle can also help.
An easy alternative is to slip two stitches purlwise to the right needle, purl the next stitch, then pass the two slipped stitches over the purl stitch.
