Purling stitches together on the wrong side with the middle stitch on top

Hi, I’m currently knitting a hat for my grandchild, who is due to be born this November.
I can’t figure out from the pattern how to do this! Purl 3 stitches together on the wrong side row with the middle stitch on top.
Purl the 1st and 3rd stitches together behind the work, take the 1st stitch off the left needle, then lift it back onto the left needle and pull the 2 stitches over it from the back; then lift the stitch onto the right needle and let the 3 stitches slide off the left needle.
Can anyone help me? 

Hi Christina, did you buy the pattern here on CP? If so, why not contact the designer? She’s the best person to explain what she meant.

You can reach her here via the contact form, or there’s often an email address listed in the pattern. 

Hi Petra 
I got the pattern from a knitting book

Books often include contact information, or you could try searching online for “how to knit stitches together.” 

Hi Christiane,
maybe the book explains how this type of decrease is done at the beginning or end.
Books often include illustrated explanations for individual steps.
Best wishes and good luck!

....I’m just wondering why a decrease with a slip stitch in the middle should be worked on the wrong side row....
I’d knit a hat in the round....
But anyway, it’s just different here...
  You know how to knit this decrease on the right side row, right? Then knit your wrong side row up to the decrease, turn the work briefly, knit the decrease as you would on the right side row (i.e., knit), turn the work back, and pick up the stitch left over from the decrease onto the needle where your “knitted” stitches are.
  I think that would be a “straightforward” solution.
Knitting together purlwise behind the work really gives you a hard time....
I still wonder why the designer does that...