1. 3V is the coil voltage. You need to put a resistor in series with a coil to get the "set voltage". Use coil resistance and voltage divider equation to find the resistor value.

 2. Here's a typical circuit for driving a latching relay:
 [![enter image description here][1]][1]

 Obviously, ensure that the transistors are only turned on for a short amount of time, as they will sink reasonable current.


 3. The relay contact is rated for mains voltage, so no problem.

  [1]: https://i.sstatic.net/kCyR6.gif