Natural convection occurs in fluid environments. Usually, it is facilitated by the buoyancy effect. It is significantly less efficient than forced convection, due to the lack of fluid motion. As a result, it is completely dependent on the buoyancy effect's strength and the fluid's viscosity. The current work investigates the convective flow of a three-dimensional Casson fluid across a rotating linear expanding sheet. The nonlinear governing equations of the steady flow were presented and reconstructed using appropriate similarity transformations. To solve the resultant equations, the three-stage collocation approach namely Lobatto IIIA was applied using MATLAB. Graphs were used to illustrate the physical properties of the required data. It was observed that while the primary velocity profile decreases as the Casson, convective, and rotational parameters increase, the secondary velocity profile exhibits the opposite behaviour. The effect of rotation, Casson parameter, and others on drag coefficient, heat transfer coefficient, and mass transfer coefficient was evaluated, interpreted, and found to be reasonably consistent with earlier research.