"I was told that if you owned a boat of 50 foot you were required to have three crew members as a minimum. Is that true."
There are no crew requirements for recreational boats, however there are minimum crew requirements if the recreational boat is a facility. The minimum number depends on the vessel length. A facility that is 50 ft in length requires a minimum of 1 coxswain and three crew members.
Source: Auxiliary Operations Policy Manual, COMDTINST M16798.3E, Section N, Table 1-1