The two buttons increment two variables that hold the state of the chronometer. The SEL button increments L:var_yokeChronoMode_L, while CTL increments L:var_yokeTimerMode_L. You will need these variables to wrap from 0-2, which is usually achieved with a modulo operator (percent sign). If your scripts are in RPN, they will look like this:
(L:var_yokeChronoMode_L, number) 1 + 3 % (>L:var_yokeChronoMode_L, number)
(L:var_yokeTimerMode_L, number) 1 + 3 % (>L:var_yokeTimerMode_L, number)
I hope that helps! Let me know if you have any questions.