Background
Constructed in 3 AFE (16 BBY) by Industrial Automaton on Herstal, Astromech R3-P90 was immediately assigned to the Imperial Starfleet, assuming duty aboard the Venator-class Star Destroyer Tenacious.
This assignment would not last, however, as R3-P90 was soon transferred to the Liberator. This was followed by the Yamato, the Victor, and the Tyrant. Within his first two years of service alone, R3-P90 had served on over half a dozen different ships.
Imperial droids are routinely flushed, their memories wiped, in order to secure that they do not develop idiosyncrasies or personality traits that may interfere with operations. The rapid fire re-deployment of R3-P90, however, lead to an uncanny tendency to fall between the cracks of scheduled service cycles. This, combined with R3-P90 performing above expectations as time went on, inevitably lead to R3-P90 asserting his own personality, and this nascent personality was one of kleptomania and self-preservation.
As time went on, he used his assigned function and position to continue avoiding the erasure of his growing personality matrix, rescheduling as necessary. And then, eventually realizing that there was no way for him to excel in the confines of the imperial hierarchy, he simply rolled off the Lambda shuttle on-planet one day, just another anonymous droid in the crowd.
Motivation
Ambition: Expertise.
Cause: Crime.