After I get armed and go to the station,
tthere exists 3 guards and they tell me that I can't leave the town.
I tried to kill the three guards
but it is no use for leaving.
The list of Day's task shows that
there is someone discovers our plan.
How can I do for this situation?:?

There is no way you can run from town - you get discovered for sure, so it's best to settle things peacefully - there won't be much ammo. ))

My guess is the player isn't allowed to leave town until all situations are resolved. Resolution will trigger the end game, most likely.

Yep. ) And the game can only be completed on day 12.

The best solution I found is to preserve your reputation and give up. The guards will be cool about it and let you go. Eva and her 2 friends never went to meet you there anyway, as you'll discover when you report back to them. Buncha goofs!