A Bianchi type-III string cosmological models is investigated in scalar-tensor Brans-Dicke theory of gravity (Phys. Rev. 124, 925, 1961). For an exact solution of modified Einstein 's field equations (EFEs), we assumed time varying nature of deceleration parameter (DP) along with relation between scale factor and time, given by a(t)=[te^t)]^1/n ,  and n are non-negative constants. It is noticed from study that the power index has their own significance on the string cosmological models. It is also analyzed that the string tension density () is an increasing function of time whereas the energy density() and the cosmological constant () are decreasing functions of time and converges to small value at present time. and n are non-negative constants.