Could these factors have interfered with the group's decision making capabilities. Did the group become polarized as it discussed the problem. More people means more information. The knowledge available to 3 people will certainly surpass the knowledge retained by 1 person. Decision making can be effortful, and complex.
More people means that areas of expertise can be exploited.
In groups, different people are good at different things. One person may be good at collecting information, another at weighing information, another at identifying alternatives. All these skills are needed for good decision making, and the chance of a single person possessing them all is less than the chance of a group possessing them. Also, you can make more monumental decisions in groups, because you know you won't be singled out for blame. Can hide, basically, from personal responsibilility.