Decreases in Trinity Stitch

I was going to make a trinity stitch scarf, but thought better of it and am now making a purse. I would like to make a triangular flap like this:

but I would like to stay in the the trinity stitch pattern:

Row 1 and all odd rows: purl
Row 2: *k1,p1,k1 in same stitch, p3tog repeat from * to end
Row4: *p3tog; k1,p1,k1 in same stitch . . .

How do I decrease and stay within pattern?

If you did your decreases on the purl rows, you could p2 together at the beginning and end. Then on the right side rows, you could work those edge stitches in stockinette with the trinity in between.

Thanks Ingrid!!