Here’s how to do three purl-side decreases.
P2tog (purl 2 together)
This is exactly like a k2tog, except you purl instead of knit. This makes a right-leaning decrease and looks the same as a k2tog from the right side.
ssp (slip, slip, purl)
This makes a left-leaning decrease and looks the same as an ssk from the right side. There’s really only one tricky bit…
First slip 2 stitches knitwise, one at a time (exactly the same as an ssk)
Slip both stitches purlwise back to the left-hand needle.
(Here’s the tricky bit), insert the tip of the right-hand needle into the back loops of both stitches from back to front. In other words insert into the back leg of the second stitch on the left-hand needle, then into the first stitch (yes, it’s a bit awkward at first, but a little practice does wonders)…
…then purl these stitches together.
dd (double decrease)
Purl 2 together, then slip the resulting stitch back to the left-hand needle.
Lift the second stitch on the left-hand needle over the top of the slipped stitch.
Slip the stitch back to the right-hand needle and move on!