Found damage on shock absorber assembly. Damage caused from the top boss contacting the shaft shock absorber shaft. The bushing was discovered missing on three aircraft at the same flight school. The manufacturer was notified of damage, affected parts replaced and the aircraft was returned to service. The quality/airworthiness department is currently reviewing work orders in an effort to isolate aircraft affected by this missing bushing. All three aircraft with missing bushings had between 1300 and 1400 hours total time.