When a misunderstanding leads Alvin, Simon, and Theodore to believe that Dave is going to propose to his girlfriend, they are less than keen on the idea, both out of jealousy and protectiveness of their place in Dave's life, and out of a refusal to have the girlfriend's son in their lives.
Alvin, Simon and Theodore get to falsely think that Dave is going to propose to his girlfriend in New York, and also dump them. In three days time, they have to reach him and stop the proposal.