Rav Moshe Shternbuch was asked (2:57) by the people of a small town who could not get a minyan together every day, if it was worthwhile to gather together to daven with five or six people or since this is not a Minyan there is no point and each one could daven alone? He answered that even three people davening together is considered a “Rabim”, a multitude which he proves from the gemara in Gitin (30a). Since the gemara in Brachos says that Hashem does not turn away the Tefilos of a “Rabim” it is better to daven three men together than alone.
He then says that the Maharam Bennet (Talmid of the Noda B’Yehuda) wants to say that it is worthwhile for even two men to daven together. The Braisa says that two men can leave a dead body alone on a boat while they daven. If it wasn’t important to daven together, then it would be better if one of them would stay with the niftar.
Although Rav Shternbuch disagrees with that proof, nevertheless he says that if you can have a minimum of three then it is better than davening alone.