Hi!
There has to be a pattern for those 3 slipped stitches on the turn if indeed that is what you’re being asked to do (You didn’t mention the pattern!)
I can see a sort of yarn forward wrap of the working yarn and either a p3tog or purling off each one separately under that wrap to create an effect.
The key is that you’re making an i-cord at the edge. Here is a case where you just follow the pattern.
Slip the 3 stitches at the end of the row. Your yarn will be in the wrong place, but purl those 3 stitches with the yarn where it is at the start of the next row. It will pull across the back to create a little tube out of those 3. Then you can continue the rest of the row from there.