You basically have to troubleshoot what is bad, the unit or the wiring.
It's simple.
At the timer , mark the wiring at the timer and remove all the wiring.
Then take a meter, or a 24v test bulb, and take a voltage reading
between station1 and common terminal when station 1 is on. You should
get 24V.
Next turn on station 2 and station 3. If you still get 24V across
station 1, you have a bad unit. If you get 0V, then the unit is good,
which means something is wrong with the wiring going from your timer to
the solenoids.
But maybe it could be something simple as programming.