Simplified model of the orbital expansion phase showing the evolution of P_d/P_c and P_e/P_d. The blue dashed line corresponds to the (3, 5, 8) 3BR for planets c, d, and e. The red dashed line represents the period ratio following P_e/P_c > 3. The blue cross is the observed period ratio and the black line represents the minimum slope of P_d/P_c and P_e/P_d which ensures successful orbital expansion to the observed period ratio.