Do you think that flannel would be too heavy. You would want your valance to have a nice crisp look. Cotton might be a better choice. Colleen

You might try laying out the three layers in order (at the end of the lenghths) and pin them together. Try folding and pinning in the box pleats, keeping the pins near the edges of the fabric if the polyester wants to "hold" pin holes. Drape this over the edge of a table to simulate a valance and see how it looks and feels. When I was making semi-tailored jackets I would do something like this with the wool and the lining to pick out an interfacing--I just had to feel and see the drape when they were together. An alternative is to take the fabrics in to a professional for an opinion. I think most of them would be happy to help.

It is not necessary to interline a valance. They are not long enough for the interlining to be of any benefit. Flannel interlining originally came from England. It was used to block draffty windows. When you are making silk drapes (full length) the interlining give the drapes more body and they hang very well.
If the fabric is a real silk it also helps prevent fading.

Bonnie