The HKB-3 hunter-killer droid was a battle droid produced by Baktoid Combat Automata prior to the Invasion of Naboo.
Nute Gunray's HKB-3 hunter-killers in action
HKB-3 droids were easily modified hunter-killer droids. They had square heads and were equipped with rapid-fire blasters. They relied on a Central Control Computer to coordinate their operation. This feature made them effective at tracking a target because they could easily coordinate with each other when sent out in groups. Individually they had poor targeting abilities, but in a group, if one droid homed in on a target, all droids in the patrol group could coordinate their fire on the same coordinates to maximize their effectiveness.
Prior to and during the Stark Hyperspace War, HKB-3s were favored by officials of the Trade Federation to serve as bodyguards and provide general sentry duties. It was during that conflict that Wookiee Jedi Master Tyvokka was killed after Trade Federation Minister Nute Gunray ordered his hunter-killer droid bodyguards to open fire on him.
By the time of Invasion of Naboo, they had largely been replaced by the highly-superior droideka. Though a rarer sight than the B-series battle droid, HKB-3 hunter-killer droids could still be seen during the Clone Wars, typically relegated to simple guard duties or patrolling remote planets.
- Scavenger's Guide to Droids