Tercer (en. Third)
/terˈθeɾ/
Slang Meanings
Third person in a romantic relationship.
Don't be the third in that relationship, it's complicated.
No seas el tercer en esa relación, es complicado.
A person who joins an already existing group.
I don't want to be the third in their trip, they have everything planned.
No quiero ser el tercer en su viaje, ya tiene todo planeado.