Transient thermo-elastic analysis of a thin composite plate, made up of a dominating matrix (region-1) and an insert (region-2), is studied because of the widespread use of composite structures in heat environments. The linked heat conduction equations for the two locations are derived using a three-phase lag heat conduction model. The composite plate's region-1 receives its heat from a step function-shaped heating source. The composite plate is expected to have lumped thermal behavior along the thickness axis. The governing heat conduction equations are solved using Laplace transformation and Riemann-sum approximation techniques. Transient impacts of thermally generated strains (found to be compressive) and deformations in the composite plate are analyzed based on the history of temperature distribution in region-1. Thermal stresses in the composite plate have been seen to mimic the behavior of the regional temperature distribution. Composite plates’ transient thermoelastic responses are investigated, along with the effects of heat source intensity, heat pulse length, region-2 volume proportion, and convection coefficients.