User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-5401949-20121021231901/@comment-3146930-20121022024918

Generally a third wheel (sometimes called a fifth wheel) is like an extra wheel, an unneeded wheel. So for example, when Jason and Piper started falling in love during TLH, He felt like the odd man out, like a third wheel. Now we have all 7 demigods together and he is like the odd man out, as he isn't really that good with people and kind of isolates himself in his machines. While he is in the group, in a way he is the most alone, the seventh wheel.