In light of the recent analysis of the second virial coefficient A2 for flexible polymers, the third virial coefficient A3 is reformulated for the helical wormlike chain, taking account of possible effects of chain ends. It is shown that at the theta temperature at which the effective binary-cluster integral vanishes, and hence A2 vanishes for large molecular weight M, the nonvanishing part A3,theta of A3, which results from various kinds of ternary-cluster integrals, becomes appreciably dependent on M as M is decreased. It is then shown that the theory may explain satisfactorily the data obtained for A3,theta in the preceding paper for atactic oligo- and poly(methyl methacrylate)s in acetonitrile at 44.0-degrees-C (theta) and in the previous and present studies for atactic oligo- and polystyrenes in cyclohexane at 34.5-degrees-C (theta). The values obtained for effective excess ternary-cluster integrals associated with the chain end beads are of the reasonable order of magnitude.