How do you estimate in the three point estimating method?

There are 2 formulas to calculate 3 point estimation.
1) Triangular Distribution E = (P+M+O)/3 ;
2) Beta or PERT Distribution E = (P+4M+O)/6 ;

where P stands for pessimist, O for Optimist and M = most likely and
PERT = Program Evaluation and Review Techniques