In real estate listings, a three-quarters bath is a bathroom with a toilet, a shower, and usually a sink. They do not contain a bathtub, because if it did, it would be a full bath. A toilet alone (or with a sink) constitutes a half bath. Logically, a sink alone would be a quarter bath, but no one uses that term.
Most usually found in older homes where they shoe-horned in a shower stall and a toilet to increase bathroom capacity.