Add 1-1/3 cups boiling water to 2 packages gelatin mixes in medium bowl; stir 2 minutes until completely dissolved. Pour into 8-inch-square dish sprayed with cooking spray. Refrigerate 25 to 30 minutes or until set but not firm.
Meanwhile, add remaining boiling water to remaining gelatin mixes in medium bowl; stir 2 minutes until completely dissolved. Stir in evaporated milk. Refrigerate 15 minutes or until slightly thickened; pour over gelatin layer in dish.
Refrigerate 4 hours or until firm. Unmold onto cutting board just before serving; cut into 4 rows, then cut each row into 7 alternating triangles. Garnish with sprinkles. Reserve gelatin trimmings for snacking.