This investigation deals with numerical treatment of heat transfer flow of a third grade fluid between two infinite parallel plates subject to no-slip condition at boundary and no-temperature jump. Three flow configurations, Couette, Poiseuille, and plane Couette–Poiseuille, have been discussed. Approximate solutions using Lagrange–Galerkin method to Couette, Poiseuille, and Couette–Poiseuille flow problems are computed and delineated. It has been substantiated that the fluid rheology and heat transfer phenomenon are greatly influenced by the third grade flow parameters, Brinkman number, and pressure gradient. A rigorous mathematical exposition of the numerical scheme is provided. Because no a priori assumptions are made on pertinent flow parameters, apart from those due to thermodynamic stability, the results presented in this investigation are also valid for their large values.