Huh? Separate wires in the air is always the worst option possible. Each logic signal must run alongside a return wire (usually ground or Vcc). Without a return wire in close proximity you have a loop antenna, not a signal cable!
Normally you'd use twisted pairs for each signal and ground for the best immunity to interference. Look at how ethernet CAT5/CAT6 cable is constructed for instance. In fact an old bit of CAT5 cable can be used for carrying upto 4 signal.