The linear and nonlinear characteristics of pressure-driven magnetohydrodynamic instabilities are examined for a high-beta ( beta T=4.5%) limiter discharge and for a typical high confinement time diverted discharge in Doublet III. These discharges are found to be linearly unstable to high-beta external kink-tearing modes even with the stabilizing influence of a conducting shell and a conducting mantle. Nonlinear calculations show that tearing mode islands are too small to overlap, possibly related to the experimentally observed stability against disruption at the discharge time of interest.