You are misunderstanding, 3MBit is your connection not your download speed. 3000MegaBit /8 =375 Megabytes will be about the max download you can get. Lower download sometime occur because of internet congestion, to many host on the network online, delay because of router and/or swtiches on your network and on the ISP network, to many other computer in your neighborhood (cable is a share medium), the server might be overwhelm, you may split the line to many time, etc. I can think of alot of possibility like maybe your not getting what you pay for.
Also if other computer download faster than yours, you might check to see if any software are downloading stuff without your permission ----Spywares