Oh sorry for misunderstanding then.
Well logic wise would dictate that chosen dignitaries never traveled alone, but how about this:
They set up base camp, the three of us enter whatever we enter and after our 'third one' betrays us we can't return the way we came, as he/she might have trapped us or knocked us out/wounded us so we couldn't follow.
Potentially the support group could be lied to by said third party or killed/sacrificed to suit his/her own purposes.
Forcing us to have to work together to brave whatever dangers we encounter.