In reply to Mathias.

Yes. If 2*b-a-c=0 then alpha=beta=0 and the beta distribution is degenerate. If you click on the link the "the PERT distribution" in the first paragraph, you will find an article that describes limitations of the PERT distribution. Included in that article is a generalization of the PERT distribution that can eliminate some of the degeneracies. See also the comment just prior to yours.