Three-point estimating is a technique "used to estimate cost or duration by applying an average of optimistic, pessimistic, and most likely estimates when there is uncertainty with the individual activity estimates."
Where tM is the most likely estimate, tO is the optimistic estimate (best case), and tP is the pessimistic (worst case) estimate, there are two different formulas for the three-point estimate, tE. The two formulas are triangular distribution or beta distribution.
triangular distribution: tE = (tO + tM + tP)/3
beta distribution: tE = tE = (tO + 4tM + tP)/6
The beta distribution comes from the traditional PERT technique and weighs the probable estimate more strongly than the pessimistic and optimistic estimates.