For almost every receiver out there, the wiring is:

Ground (black or brown wire) closest to edge of receiver
Positive (red or orange wire) middle of the three
Signal (white or yellow wire) furthest from edge of receiver

If the plug has the little ridge on it (like Futaba plugs) the signal wire (white or yellow) goes closest to that ridge, the ground (brown or black wire) goes furthest from that ridge.

Old Airtronics receivers had a different wiring order, but they haven't made them that way for years.