Do both of your computers have USB 2.0 ports? If the adapter is connecting to your computer at USB 1.1, it will only operate at 11Mbps.
How far apart are your two computers? Walls, etc., in between?
You are unlikely to get 3MB/sec (24Mbps) out of any 802.11g connection. You MIGHT get 2.5Mb/sec (20Mbps) if all the planets align and your computers have no obstructions between and are close to each other. If you are actually getting 1.5 - 2MB/sec (12 - 16Mbps) measured throughput, that is not too bad for 802.11g real-world througput. You might be able to nudge it up some. You're connected faster than 11Mbps in any event.
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