Each Space Shuttle orbiter carried three main engines.
They were RS-25 engines, commonly called Space Shuttle Main Engines, mounted in a triangular arrangement on the orbiter’s aft fuselage. The engines burned propellants supplied from the external tank and provided thrust throughout the powered ascent.
The three engines were started shortly before launch, ahead of the solid rocket boosters, allowing controllers to verify their performance before liftoff became irreversible. Their nozzles could gimbal to steer the Shuttle by redirecting thrust.
A common mix-up is counting the two solid rocket boosters as main engines. The boosters added most of the initial liftoff thrust, but they were separate propulsion units. Another distinction is reusability: the RS-25 engines were removed, inspected, refurbished, and flown again after missions. The Shuttle program used 46 reusable RS-25 engines across its flights.