Definition
Of a decidedly lower quality or condition.
How People Actually Use It
A third string is the backup to your second string booty call hoe. Definitely unwanted, they're an easy and guaranteed lay.
"Fundo's second string hoe was busy pig roasting, so Fundo called up his backup to his backupe. Though he didn't really dig her like that, he knew that she was a third string hoe and that anything goes."
Source: Urban Dictionary