Pulsating behaviour is observed in laminar flames of different kinds ranging from edge flames in a mixing layer to premixed flames even in partially premixed triple flame. This study focuses to study the physical mechanisms behind the pulsating behaviour of triple flames. A numerical simulation is performed for a typical case of high Lewis number and low Damköhler number, where pulsating instability is observed. The flame undergoes a limit cycle oscillations. Last cycle of oscillation is analyzed to study the physical mechanisms. Temperature, and concentration filed data is plotted for different phases of the cycle. Vector plots representing the heat and mab diffusive flux gives further insight into the flow of reactants into the flame zone and also the flow of heat away from the reaction zone. Also a closer look at the field variables at the triple point is made. With the help of these plots, some insight is obtained in understanding the underlying mechanism driving the thermo-diffusive oscillations.