Three handicapped students and a teacher's aide were injured when a Prince George's County school bus struck them in the Louise Cosca Regional Park in Clinton yesterday morning, said county police.

The youths were among more than 125 mentally retarded students and their teacher's aides on a field trip from the Hillcrest Heights and Shady Grove special centers, a police official said.

Fernando Viteri, 18, of Forestville, the most seriously injured, suffered multiple fractures and cuts and was listed in critical condition at Washington Hospital Center, a hospital official said.

Three others were taken to Southern Maryland Hospital. Karen Ann Hardebech, 20, of Fort Washington was treated for a pelvis fracture and listed in serious condition. Robert Goodman, 20, of Oxon Hill suffered a leg fracture and was listed in good condition.

Ernest J. Bannister, a 16-year-old teacher's aide from Hillcrest Heights, was treated for minor cuts and released yesterday afternoon.

Gloria Savage, 44, a 13-year veteran bus driver, was listed in stable condition after being treated for shock at the Clinton Community Hospital.

Savage was driving one of the buses transporting the children to the park. All the students had gotten off the bus when she realized that the exhaust pipe of her bus was embedded in an embankment, police said.

When Savage pulled forward, the bus lunged, slowed and then surged forward again, running off the parking lot over a small embankment, knocking down shrubbery and crossing another parking lot, before striking the four youths, who were walking nearby, police said.