Beat all but 1/4 cup of the sugar with the egg whites until stiff. Spread a thin layer of the frosting on a cone, then, working from the bottom up, overlap cookies, using more frosting as necessary. Repeat for the other 2 cones. Use a small sieve to dust the trees with the remaining sugar.
Note: Straight pins can be used to hold the cookies in place until the frosting dries, but don't forget to remove them.