I assume you mean the 3 inch strips are vertical from the waist to the hem? You can tack them down at one or two points using a small stitch in matching thread to the tulle. If the strips are layered, just tacking one of the top layers to the adjoining top layer strip should be all you need. You might want to consult with a seamstress or tailor if you do not have much sewing experience.
tape it down