Axially loaded reinforced concrete walls can be constructed with various support conditions. Walls can be designed using simplified design methods given in codes such as the Australian Concrete Standard (AS3600-09), the Eurocode 2 (EC2-04) and the American Concrete Institute Code (ACI318-14). There is little research published on the behaviour of three-side restrained (TW3S) walls. As such, an experimental programme was undertaken on three TW3S walls. The dimensions of the walls were selected by incorporating the provision for varying values of slenderness ratio and aspect ratio. All panels were subjected to eccentric axial loadings and tested to failure. The design equations specified in the aforementioned codes were found to be inadequate in predicting the failure load for TW3S walls of high slenderness ratios. In addition, an instability analysis incorporated into a computer program called WASTABT, to establish the behaviour of TW3S walls, is also presented. The outputs from the WASTABT program were compared to test results to verify the material and geometric modelling techniques adopted. Finally, this computational approach was used in conjunction with a probabilistic, semi-empirical design procedure specified in the Eurocode 0 (EC0-02), leading to more realistic design capacities of slender TW3S walls.