Longer Answer: There are lots of limiting factors at play here. For various reasons a "56K" modem is almost always restricted to a real maximum of closer to 53Kbps. The biggest limiting factors are the quality of the local circuit and the distance to the PTT's switching office. Neither of these can be tweaked with "&" commands. You also need to know that not all of your bandwidth is available for straight through file transfer. There's a lot "overhead" with a file transfer (for example) with "hand shaking", "Acknowledgements", "Flow Control" and so on. And finally (sort of) the connection between your dial-up server (your ISP) and the sites into which you're connecting may be crowded or slow. Again, "&-anything" is not going to be effective.
If you're averaging around 3Kbps download, that is actually not too bad (believe it or not).
Your only real hope, is to look into an upgraded circuit, if available in your area. DSL is the current hot-ticket. Cable TV suppliers also have an approach. Talk with your ISP and your PTT and see what else they have. be prepared to spend a bit more on your connection too.
"Can't have the performance of a Supra if you own an Echo!"