R3

R3 can refer to either a specific astromech droid model or one of several individual astromech droids.

Droid models
R3-series astromech droid R3-D-series astromech droid

Individual droids
R3-A2 R3-D3 R3-K8 R3-S6 R3-T2 R3-T6 R3-T7 R3-Y2