If three wire was installed in 1955, or even up to 1995 and done in compliance with code at the time, it can stay that way as a general rule.
If the circuit were installed after 1996 code went into effect, it doesn't matter it should have been 4 wire. If service upgraded or circuit was extended after 1996 code went into effect, harder reading of details needs consideration. Often it will need upgraded to 4 wire though.