Socket 939 Opterons certainly are overclockable. As already mentioned, you just have to do it via FSB since the multiplier is locked (you can only lower it, but not raise it). As long as you monitor your temps, you should be good to go to at least 1.4v even on mediocre cooling. Most 165s (1.8GHz stock) were capable of reaching 2.8-3GHz back in the day, so 3GHz probably be a joke for a 185 since they are already at 2.6GHz.
Keep in mind that these CPUs are not as temp tolerant as let's say Intel's Haswells which can easily run 24/7 stable in the upper 70s... Socket 939 Opterons like to stay under 60*C.
Also keep in mind that when you're OCing via the FSB, you are also overclocking you RAM. So if you have DDR400 RAM which runs at stock FSB of 200MHz, you either need to change the FSB:RAM multiplier, or adjust your RAM timings/voltage accordingly in order to achieve a high FSB overclock.