Debbie Bliss Moss Stitch Rib Hat - help!

I’m having trouble understanding part of this pattern (apologies for copying and pasting, wouldn’t let me put a URL in first post!)

"Moss Stitch Rib Hat
Debbie Bliss
SIZE
To t an average sized head.
MATERIALS
Two 50g hanks of Debbie Bliss Bliss Paloma in Jade 28. Pair each 9mm (US 13) and 10mm (US 15) knitting needles.
TENSION
13 sts and 18 rows to 10cm/4in square over patt using 10mm (US 15) needles.
ABBREVIATIONS
cm = centimetres; dec = decrease; k = knit;
p = purl; patt pattern; psso = pass slipped st over; rem = remaining; st(s) = stitch(es); tog = together.

TO MAKE
With 9mm (US 13) needles, cast on 57 sts.
1st row (right side) [P2, k2] to last st, p1.
2nd row [K2, p2] to last st, k1.
These 2 rows form the moss st rib.
Work a further 10 rows.
Change to 10mm (US 15) needles.
Cont in patt until hat measures 25cm/10in from cast on edge, ending with a 2nd row.
Dec row P2tog, [k1, p3tog] to last 3 sts, k1, p2tog. 29 sts.
[B]Rib 3 rows as set[/B].
Dec row P1, [slip 1, k2tog, psso, p1] to end. 15 sts. Rib 1 row.
Dec row P1, [skpo] to end. 8 sts.
Break off yarn thread through rem sts, pull up and secure.
Join seam.
Make a 5cm/2in diameter pompom and sew to
the top of the hat."

I’ve done the first decrease row, which starts with purl stitch, but then it says “Rib 3 rows as set”.

The beginning of the pattern is 2 rows alternating to create the rib, purl first row 1 and then knit first row 2. To “rib 3 rows as set” do I do row 2, row 1, row 2? Or the other way around? I feel like I should do row 2 after the decrease row so that it goes back to a knit first row but not 100%. And does that mean 3 rows or 6 rows aka 3 “sets”?

Thank you for your help in advance!

Welcome!
The dec row counts as a row 1.
So yes, do row 2, row 1 and row 2 again.

Thank you! That makes sense, I just wanted to be 100% sure!