the failsafe numbers that you set, start at 0, so if you have 3 flight modes, you select them with 0, 1, 2.
You can actually get 4 flight modes from a 3 position switch if you are very sneaky and make your failsafe for that channel go to a place that you can't get to from any of the switch positions. Of course you have to set up the FC to have 4 flight modes.
I did this on an old radio with a 2 position switch to make a 3rd position. I can now switch the transmitter off to make it do RTB.
Now you want to do this, but be careful. I bet you didn't think about fancy transmitters that won't let you switch on with the throttle stick in the middle, or with switches set a certain way. You need to turn this stuff off because you may need to switch out of RTB quickly.
Also, you must be more careful about switching off vehicle then transmitter. You may have gotten away with switching transmitter off first before, because it would go to failsafe which by default stops all motors, but now when you switch the transmitter off, the motors will start up and it will try to fly away and cause BAD problems.