Help please

please tell me what P2SSO means and how do I do it.

p2sso means to pass 2 stitches over.
Slip 2, knit 1, pass slipped stitches over.

p2sso means to pass 2 stitches over.
Slip 2, knit 1, pass slipped stitches over.

And here’s a video showing you how to do it: http://www.knittinghelp.com/video/play/slip-k2tog-pass-continental

do i use 3 stitches?

Yes, it decreases 3 stitches to 1 - slip 2 (as if to knit 2 tog), k1 then pass both slip sts over together. It’s called a centered double decrease.