Beat eggs, one at a time, into the feta cheese, cream cheese and dry cottage cheese. Blend well. Set aside.
To make triangles: Cut filo into 3-inch strips before opening. Wax paper will still be on. Open one strip and cover the rest to prevent drying. Put 1 teaspoon of the cheese mixture in the bottom right hand corner of the buttered strip and fold over into a triangle shape as though folding a flat. With each fold, make certain the bottom edge is parallel to the alternate side edge. Lightly butter finished triangle. After all the filo strips have been made into triangles, place on a cookie sheet and bake at 400 for 15 to 20 minutes, or until brown. Serve warm. Cheese triangles can be frozen and reheated for party use. The unbaked triangles may also be frozen and baked when needed.