Remember that, even if you issue a warning, you still have to follow a fair procedure. This does not, necessarily, have to be a formal disciplinary hearing, but the employee must be given an opportunity to state her case to avoid receiving the warning. If there is a reasonable explanation for the conduct and no warning is warranted, then the employer should not issue a warning. If you don't follow a fair procedure before issuing the warning, you might as well chuck the warning in the fire.
I suggest that you do get a formal disciplinary code drawn up, in which offences are described in broad terms or in detail if you want and next to the offence, you list the punishment, i.e warning, immediate dismissal etc. That would be handy for future use.
When you want to dismiss an employee that is under probation on operational requirements pertaining to her ability to do the job, you also, still have to conduct a disciplinary hearing. You would have to show the presiding officer that you have done your part in trying to get this employee up to speed and that, despite that, she is still not where she should be three months on in the job.
If, however, the employee is found guilty of misconduct on a dismissable offence within the three month period, there is nothing that prohibits you from firing her within the three months. So, for instance, her lying and staying away from the job etc would be things you would already be able to discipline her on and that counts towards the warnings etc.
IF it is on her contract that you may dismiss her on two warnings, then you would be able to dismiss her when you have issued two warnings, but please, have regard to my comments regarding fair procedure above.
If she goes to the CCMA (and she probably will, regardless of whether you use fair procedure or not) for unfair dismissal and you have not followed fair procedure, the merits on why you fired her, would not count.