The marginal pressure profile that is imposed either by ideal ballooning modes or by interchange modes on every nested flux surface of a three-dimensional equilibrium modelling the ATF device is determined for conditions of vanishing net toroidal plasma current. The corresponding limiting values are β = 3.75% and β(0) = 9.89%. Mercier modes dominate in regions of strong stellarator-like magnetic shear (ι'/ι > 0), and non-symmetric ballooning modes (non-symmetric eigenstructures in the extended poloidal angle domain) dominate in regions with strong tokamaklike magnetic shear (ι'/ι < 0).