A third string is the backup to your second stringbooty call hoe. Definitely unwanted, they're an easy and guaranteed lay.
Fundo'ssecond 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.
Distantcontacts when making certain illegal purchases. Usually utilized when there's a shortage in your immediate supply, causing you to have to call people you normally avoid to score from.
When one who is inexperienced and/or not completely fit for the task at hand is given a chance to try it without assistance from a more experienced person, they are being given their deserved third string solo.
Phil: I'll be away Friday, so the intern will have to finish the project proposal.
Clyde: But he's only an intern! Shouldn't George help him with it?