While no one is too sure why the "triplationship" occurs, it is usually characterized by the increased amount of time spent together as opposed to the other travelers and even sometimes physical contact.
The usual end of the triplationship is brought by the method of standard rejection (i.e. "I'm not ready for a relationship, I think we should be friends, etc.) with one party not really understanding why. The after effects of the triplationship range from completely awkward social avoidance to completely normal return to behavior before in the home environment.
"Nah it was a triplationship, over before we landed."