We study the contribution of charged Higgs boson to the rare decay of the quark topt→bWZin models with extended Higgs sector that include doblets and triplets. Higgs doublets are needed to couple charged Higgs with quarks, whereas the Higgs triplet component is used to obtaing a vertexH+W−Z.It is found that this decay mode has a large branching ratio that may allow to test the custodialSU(2)csymmetry. ©1999 American Institute of Physics.