Wireless file transfer speeds seem to be in that range. Your wireless throughput with 3Mb is about 24Mbit.
I am not sure at what percent is the usual overhead associated with the wireless signals since i'm not a dlink engineer, and no one has been forthcoming to answer an old question on how it's related to the dlinks switching capability.
I am just going to go on a whim here and say that it takes part of it's ~4Gb switching capability and most likely a portion dedicated to wireless so the swtich doesn't get bogged down when trying to handle traffic that may be using the gigabit ports.
24Mbit seems to probably be average, since it falls close to what I was seeing when trying to wirelessly stream HD content. Though in my case, I only saw a max of ~18Mbit throughput on a seemingly 300Mbit link before the content started to freeze beyond buffering.
I am not going to make any suggestions since I am sure you have exhausted all of them anyways. Well, ok, if you REALLY wanted to experiment, you could first see what other AP's are running in your area and what channel they are operating on. Hopefully you can find channel 1, 6 or 11 decently free. Downgrade to f/w 1.2 I believe, which then you will be able to force 40Mhz only, compromizing the "good neighbor" policy. If you wanted to go even furthar, you can break telecommunications/radio laws and flash the dlink with an international f/w and download the drivers for the card as well, and choose a higher channel than 11.
Otherwise, your just going to have to learn to co-exist with the shady wireless, or try a 5Ghz AP to see if that helps.