Which one of the following equations could be used to calculate average or mean values(eg. PERT, Rule of 6) for each parameter?

Mean value = [Optimistic value + (Most likely value) + Pessimistic value]/3

Mean value = [Optimistic value + 3(Most likely value) + Pessimistic value]/6

Mean value = [Optimistic value + 4(Most likely value) + Pessimistic value]/6

None of the above