It depends on the employer regarding how long you have to wait to hear from the job offer after the third interview.
If the employer needs immediate hiring for their job opening,
I think they should call you as soon as possible for at least 2 days and a maximum of 5 days.
if the company does not need immediate hiring of a vacant position,
I think it will take you 2 to 4 weeks.
if there are lots of potential applicants for those positions you might end up not being hired or worse case scenario,
you will not be notified that they have already chosen another applicant.
When an applicant reaches that stage of interview there are lots of chances that you might get hired because it means that you are qualified and are best fit for the position.
So be prepared for the next step,
they might call you within 2 to 5 days after the 3rd interview.