Decorative panels? Four would look best.

We agree that four panels would look best, however, two (one on either side) will unify the three windows, sending the illusion that there is only one large window there. Be sure and hang them at least six inches above the window to create height in your room, therefore giving it a grandness!

I think you could also do two. Hang the rod high and make the panels full, keeping them off the windows.