It refers to the third person who is hanging out with a couple. Often times the couple might invite the third person to hang out with them out of pity. When this happens, the third wheel is usually left out of the decision making process, and tags along with what the couple has decided and planned.
We are all familiar with the story of Adam and Eve. The serpent comes along and tempts them into eating the fruit from the tree of knowledge of good and evil, the very tree God told them not to eat from. Once they do,
They both saw clearly and knew that they were naked. (Genesis 3:7a)
A conversation takes place. A decision is made. All by the couple. While God was a part of the scene in the verses before, now God is reduced to being the third wheel. There along for the ride, but left out of the conversation that leads to a decision.
How true is that in our own lives?
We carry a Bible with us, a cross in our pocket or around our neck. God is toted around with us, but is that all we do? Do we forget to include God in the decision making that we do on a minute-by-minute basis?
The couple is riddled with guilt after they commit the act they knew they were not suppose to. This act of eating the fruit of the tree is theologically referred to as, “The Fall.” It is referred as such because in that moment humanity for generations to come were separated from God, a result of God being treated as the third wheel.
Sometimes the third wheel feels uncomfortable when the couple cuddles or sits close on the couch. It gets awkward for the third wheel.
But not for God. God wants to be a part of what we are doing. God wants to a part of our lives. It is up to us to let God be more than just a cross in our pocket or around our neck.