If I were looking to invest in a startup, especially one by college kids, I can see the preference to having 2-3 cofounders. Because it reduces my risk as an investor in the case where one guy gets hit by a bus, and ensures that the cofounders have somebody they can ask for advice who is intimately knowledgable about and vested in the success of their venture WITHOUT necessarily having to lean on me, the investor, all the time -- because my time is finite and to further increase my success chances and happiness I want to spend time on many ventures, plus family and friends and life chores.

As an entrepreneur my bias is to single founder, precisely because that also reduces my risk. and maximizes my time efficiency.

Summary: which number is best depends on your role, whether you are the entrepreneur or merely the outside investor.