If you have a job scheduled for 2pm, 4pm and 6pm, the job starts at 2pm is still running at 4pm and finishes at 4:15pm, the job wills start again at 6pm.

If the job is still running at the time it is scheduled to start again, SQL Agent won't start it. It also won't 'delay' that schedule. Instead the job will start at its next scheduled time.