A good summary here on warnings, process and reasons:
Interestingly, many people still think that after three warnings you can be dismissed summarily following the next misconduct. That is of course not the case. There is no such ‘three strikes’ rule in employment law. Every warning has to be looked at indiviudally and a dismissal based on a warning or a series of warnings then also has to consider all the circumstances of the case. But repeated warnings for similar conduct can establish a pattern an employer can rely on. One warning for serious misconduct can be enough to justify dismissal even if the next incident is below that threshold.
The tricky bit with warnings is to not confuse misconduct with poor performance. Poor performance can justify a warning, when the employee is just careless, lazy or sloppy. They could do better, but they just dont want to. A warning is useless, however, if the employee is incapable of doing the job. They try to meet the required standard, but they are simply not good enough. That’s incompetence and requires a different approach. You cant warn an incompetent employee into competence. The mistake was made at the hiring stage and all an employer can do now is to provide training and supervision. Performance improvement plans are required and if they show no effect, a dismissal is probably justified. But not if the employer relies on warnings instead.