Here is how I would describe the job using 3 colors of wire and switches #1, #2, #3
REMEMBER: you must test this using a "1/4" stereo cable" not a regular guitar cord. The plugs on the stereo cable have a sleeve, tip & ring. Plugging a guitar cord into a stereo jack shorts the ring to ground.
Identify the ground lug on the jack, that is the one that connects to the sleeve of the jack. The tip lug is the one that connects to the long spring contact of the jack. The ring is the remaining one, that connects to the shorter spring contact. The most common mistake is to mis-identify the lugs on the jack.
1. Solder a black wire to the ground lug of the jack. This is your ground wire.
2. Solder a black wire to one lug of each of the three switches.
3. Connect all the black wires together for this I would just use a wire nut. Now all your ground wires are connected.
4. Solder two red wires to the tip lug of the jack.
5. Solder two green wires to the ring lug of the jack?
6. Solder a red wire to the remaining lug of switch #1
7. Solder a black wire to the remaining lug to switch #2
8. Now you have a red and a black wire left over. Solder both of them to the remaining lug of switch #3.
When you press switch #1 you are grounding the tip signal. When you press #2 you are grounding the ring; press #3 you are grounding both the tip and the ring.
sorry I don't have a schematic for you but if you follow these instructions everything should turn out great.