  The nonzero widths of heavy particles become significant when they appear in
the final state of any decay occurring just around its kinematical threshold.
To take into account such effects, a procedure, called the convolution method,
was proposed by Altarelli, Conti and Lubicz. We expand their study which
included only threshold effects for t-> bWZ in the standard model. We discuss
finite width effects in the three body decays t-> cWW,cZZ and A^0(h^0)->tbW in
the type III version of a two Higgs doublet model. In particular, we find a
substantial enhancement in the decay t->cZZ, which brings its branching ratio
to BR(t->cZZ)~10^-3, and in the decay A^0->tbW, which, unlike the h^0 case,
becomes competitive with the A^0 two-body decay modes.
