An extremely unattractive lady. A reference to the practice of putting a bag over the head of an ugly lady while having sex with her. A three bagger is so ugly she requires a bag for her, a bag for you and a bag for the guy looking in thru the window.
Geez, check out that one at the bar, she's a three bagger!
A woman uglier than a two bagger. One bag for her head to stop you seeing her ugly face. A second bag for your head just in case her bag falls off. A third bag containing Viagra so that you can still get it up in case the other bags fall off!
Tracey was such an ugly cow that she was a three bagger. One bag to cover her ugly mug, another to cover mine in case hers fell off and another full of Viagra so that I'd get hard!