The output of generate is in GeneralizedBetaDistribution.Pert is giving just the value 3 for symmetrical PERT and the reason, I believe, is below:
In GeneralizedBetaDistribution.cs Generate calls Random in the order (min, max, alpha, beta) Random on the other hand accepts the parameters in the order (alpha, beta, min, max) as such parameters from Generate are being mapped in the following order
min->alpha
max->beta
alpha->min
beta->max

In a symmetrical distribution as the underlying distribution is B(3,3) so the random variable is being generated as Beta(min,max) and then scaled to just the number 3.

double[] Generate(int samples) has the same problem with the parameter ordering.