A two-step method of derivation of conduction z-transfer function coefficients for three-dimensional wall assemblies is described in this paper. Results of the calculations are presented for clear walls listed in ASHRAE Research Project 1145-TRP and some other two-core masonry block wall assemblies. Overall resistances, three-dimensional response factors, and structural factors have been computed using the finite-difference computer code. The z-transfer function coefficients were then derived from sets of linear equations, which included relationships with the response factors and compatibility conditions. These equations were solved using a minimum error procedure. Very good agreement between heat fluxes calculated using three-dimensional response factors and three-dimensional z-transfer function coefficients was achieved. Additionally, wall time constants, which are important for hot box test measurements, were developed from the asymptotic values of the response factors’ ratios.