Modern aerial threats are characterized by increasing speed, range, and manoeuvrability, presenting significant challenges for defence systems, particularly at high altitudes where low atmospheric density limits interceptor control authority. This paper addresses the interceptor trajectory design problem by formulating it as a non-linear optimal control problem solved via the Gauss Pseudospectral Method (GPM). The proposed framework utilizes a three-degree-of-freedom dynamic model. It incorporates phase-dependent aerodynamic, propulsion, and control constraints to ensure mission realism. Various configurations are investigated: single and multi-stage interceptors, the last ones equipped with double and triple-pulse propulsion systems. Numerical results demonstrate that the GPM-based approach generates feasible trajectories following an optimized ascent-descent-ascent profile. This maximizes manoeuvrability by leveraging denser atmospheric layers while complying with all constraints. A comparative analysis reveals that while triple-pulse configurations enhance high-altitude control and interception flexibility, they feature a total range reduction due to increased structural mass and lower propellant fractions. These findings confirm that GPM-based optimization is a reliable tool for planning complex interception missions. The framework shows promise for generating reference trajectories to be followed by real-time on-board guidance, improving defensive capabilities against emerging high-performance threats.