Tom DeMarco describes a method in Waltzing With Bears that's quite interesting: use a triangle defined by the three points as a rough approximation for the prob

Tom DeMarco describes a method in Waltzing With Bears that's quite interesting: use a triangle defined by the three points as a rough approximation for the probability distribution, and then cumulative distribution function calculations are just juggling quadratics that you can plug into Excel. Mind you, these days Excel's got the distribution function formulae in it that you'd need to do it "properly" with a lognormal distribution, but I like the way the three estimation points have a direct and obvious link to the result with the triangle method.