You only need one channel and three output pins on the Arduino. Call them A, B and C.
Program 1000-1100 to make A high, and B and C low.
Program 1400-1600 to make B high, others low.
Program 1900-2000 to make C high, others low.
That works well with a three position switch on your transmitter.
Presto camera select.
a7