Lets assume that the three horses are tied at point A, B and C of the triangle ABC.
The sides of triangle are 30m , 40m and 50m and the length of the rope is 7m
Area grazed by three animals = The sum of the areas of 3 sectors with central angle θ1 , θ2 and θ3 and 7m radius
Thus the area gazed by horses is 77 m2.