Also you might want to try an MCR key, from what you are describing, ACD will probably not fit.
With an MCR key it will ring on all 3 phones at once. When the first person picks up, the other two phones are freed up to accept another call on the same number, and so on. It is set up the same as a standard ext except enter MCR instead of SCR. This can also be forwarded (FDN and HUNT) to its own mailbox with a password that all the people know so anyone can pick up the messages. If you don't want it to go to vmail you can set up a dummy phone for the MCR with no forwarding, this will cause it to ring forever, if thats what you want.