do two arduinos that are linked to a common 9v rail and ground still need a common negative connection for a serial tx rx connection? so that there a 2 wires rather than 3 to connect between the two for purposes of serial communication.  And the same question for 3 arduino megas on a common rail and using serial connections?
All voltages are ground referenced, hence the ground is the only (supply) connection required. Remote modules can be self-powered and signals will still be discriminated as long as ground is connected.