If you have the capacity on the Legend, how about 4-two line hunt groups. Have the 2 lines for each person "owned" by their extension, so they can receive 2 calls at a time, or they can be on one line, and ingore another incoming call, allowing it to go to voice mail. You still have a problem with the third caller getting a busy signal.
Or set up a small two or three line hunt group. Don't put the lines on any of the phones, but have them be answered by one of the automated attendants. Use selector codes to transfer directly to mailboxes. Each private line has call forward on busy to the pilot number of this hunt group. 1st call rings at the extension, 2nd call gets "for Fred, press 1; for George, press 2; etc." and the call goes into the mailbox instead of ringing on the busy extension.