Usually people use little b to denote bits and big B to denote Bytes to help with this kind of communication.
A 3Mbps connection is a theoretical maximum connection of 375KBps. Taking into account overhead, downloading something at 300KBps is reasonable for a 3Mbit Internet connection.
With my 7Mbit Shaw link, I rarely download over 820KBps even though my theoretical maximum is 875KBps.
http://www.dslreports.com/stest
Good place to do a speed test.