Abandoned Cart #3
You probably want to wait at least two days after the second abandoned cart email to send the third one. The third can be a good time to mix up the tone of the email / subject line to try and catch them off guard.
Perhaps something more playful or informal.
I know that people are always hesitant to bombard their customers with emails and annoy them, but remember that they can always easily opt out if they want to.
And most of the time, people are just busy, and could use the reminder. You can always look at unsubscribe rates and adjust if they're getting high.