Sqlite net multi tables

Hi,
I have some problems with the relations between my 3 tables. The 1 is "StatsEx" many to one => "Ex" many to many with "Trains". My table "Ex" is filled when the app is launch. But during the user utilisation, I would like to create new StateEx or Trains and update only the link with my table Ex.
Can you help me ? Because I don't know how to do it without update also my all table Ex.
Quent.