Being the third wheel is read as feeling left outside a closeness others share. Dream books link it to a fear of being in the way, or to a wish for a bond of your own.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.