How close can I bury circuits to each other?
If this run is going to an  outbuilding and will be inspected, you can't have more than 2 circuits without a sub-panel. If an outbuilding, you need to run a 4 wire (2 hots, neutral and ground) to a sub-panel, add ground rods at the structure, and insure the grounds are isolated from the neutral. Running fewer wires (the 4 wire feed) feeding a sub-panel might actually be less expensive than running 3 separate circuits (which isn't code legal for that long a run anyway) and give  you a much more flexible/expandable setup going forward.