D1 = {1:40, 2:70, 3:70}
D2 = {1:40, 2:50, 3:60}
D3 = {1:70, 2:80, 3:90}
all_dict = {**D1, **D2, **D3}
subject_3_values = [value for key, value in all_dict.items() if key == 3]
highest_mark = max(subject_3_values)
print(f"The highest mark in subject 3 is {highest_mark}.")