Please help me out.
can not send train to a proper station with signals. It follows the default schedule
Scenario for test:

There are 3 stations: a, b and c.
I have mapped them through station mapping a=1, b=2, c=3. Each Smart stop signal output (combinator at the stop) shows their mapping correctly.
I have created a simple line a> b> c (with wait 3 sec at each station). To test the forwarding of the train lets tell the station 'a' to send the train to 'c' instead of 'b' as it is scheduled by default.
For this, I place a constant combinator at the station 'a' (mapped as 1) and send the signal Signal Station # = 3 (to send it to the station '3' which is station 'c') to smart stop signal lamp of the station 'a'.
No result so far. The train follows the default schedule.
The UI shows Station Leave when time: 3s: go to signal # or go to # 3

• The line is checked as active in Rules.
• Use station mapping is checked
• Signal checkboxes are checked.

I also tried to do the other way. Mapped all stations and in set rules so that after station a the train should go to the station c ('3').
Now, if i click on the train the UI says that Station Leave when time: 3s: go to #3 (mapped #3)
But train will still not go to the station 3. it goes 1>2>3.
What am I doing wrong? :(

If the train leaves because the waiting time is reached it will always go to the next station, no matter what (i don't remember why i decided to do it that way, but it is intentional). So for any of the rules to work the train has to leave for another reason (empty/full cargo, circuit condition, etc).

Also make sure to set the signal in the lamp to the signal that determines where the train should go (you don't have to use the "Signal Station #" signal.)
Here's the settings i used to make your example work:
My constant combinator sends the green signal = 3 to the train station and the lamp (the condition in the lamp is also set to the green signal, doesn't matter if the condition is true, the lamp is just to tell SmartTrains which signal it should check for).
So when the train arrives at A it immediately goes to C.