I don't have much experience with hyperthreads, so I would say to experiment with it to see how many you can run without a performance hit. People posted some good benchmarks here for multi-threaded Robocode on their systems, some with hyper-threading: User_talk:Voidious#CPU_benchmark_advice_47. You may well be able to run 5-6 without any issues.
But 3 on each system is already quite a few rumble clients. :-)