The easiest way would be assign multicalls of the number on the other two phones (assuming that the first has it as the prime), set them to ring and that is the end of that. You would need to look at the call forward no answer timer to reduce the number of rings (in COS). I am not sure what you are asking on the last part, are you looking for a message waiting indication on each phone? Then build a message waiting indication button for this mailbox on each phone and it will work.