Three-point estimate is a tool used to help increase the accuracy of cost and time estimates. Like the name implies, there are three parts which are the three different estimates.
An optimistic estimate is what you hope will be the amount of time to complete their project assuming everything goes as planned.
The pessimistic estimate is the worst case scenario and is an estimate that you give if everything goes wrong.
The most likely estimate is what will most likely happen and usually falls between the pessimistic and optimistic estimate.
Then use that in the formulas to give an indication of actual finish time and potential deviation