My 2 cent on this is that partial-parties that DF 1 or more members should have a shared vote between the members of the pre-made. For example, in your case, the three of them would cast their vote and based on the majority, that would go to a single vote, matched with your own. If two of them voted yes and one voted no, it would be a total of 1 vote for yes between the three of them against or with your vote regardless of your decision. This would prevent parties from over powering dungeons like this, and especially CT for vote kick or vote abandon abuse.

The reason I suggest this is because usually pre-made parties are easily influenced by a single member of their party or go in with the intention of abandoning after a specific point (like the two boss clear).