This stitch is just a plestrina stitch with an extra knot on each knot. This makes the knot look bigger and fuller.

You need to know the palestrina stitch to be able to do this stitch.

triple_palestrina_stitch_1      triple_palestrina_stitch_2
Fig 1: Lay one palestrina knot on the stitch line as illustrated.
Now make another similar knot over this knot.
  Fig 2: Take the needle beneath the base stitch without plucking the fabric underneath or the threads of the first knot to make the second knot.
triple_palestrina_stitch_3   triple_palestrina_stitch_4
Fig 3: Continue this process of making such knots.   Fig 4: A finished line of triple palestrina would look like this.