I hope I can help explain it in a way that makes sense.
Purl the 3 stitches together and leave the 3 stitches on the left needle, but the new stitch will actually be on your right needle. Wrap your yarn around your right needle to make the YO, then insert your right needle into the 3 stitches on your left needle that you already purled together once and purl them together again. On this final P3 together you'll remove those 3 stitches from the left needle. The result is that after this sequence you'll have 3 "new" stitches on your right hand needle.
Any other questions, please don't hesitate to ask