Yep, there were three on mine. One at each end of the channel and then one to hold the channel on the top at the opposite end. You'll find them when you take your old stuff out. I tried unscrewing mine on the first window but by the second window, I saw it was easier to just tear out the old channel and try to remove the screws later.

Lock ring pliers. http://www.mactools.com/ShopOnline/Product/tabid/120/productid/319702/variantid/310461/Default.aspx