What is the chance that a 29 yr old female has pituitary cushing's a third time?

Small but there: 12-15% of those with cushing's disease can have recurrences after initial surgery. After one recurrence the risk of having another increases somewhat. Even following surgery +/- radiation there is no guarantee that each cell of the adenoma is gone. Those left may grow into a new adenoma. Other treatments may be warranted. Access to a pituitary endocrinologist may be helpful.

Cushings happens: It is not common, but unfortunately it happens. In good hands, a first surgery for Cushings will be successful about 70-80% of the time. But sometimes the tumor is not discrete, can be difficult to identify and reach. Are you sure it's pituitary, and not ectopic? Most importantly you should be followed by an Endocrinologist and Neurosurgeon experienced in tough cases like this.