I was after some hardboard, but my local store says the only have 3mm MDF.
  I want it to level a floor.When I have used hardboard in the past I give
it a brush with water , then leave to dry before putting on the floor.
 Is 3mm MDF suitable for the same job? If so do I need to prepare it in any
way. I`m told it is smooth on both sides , so I don`t know if treating with
water will be any good with  it?

 Thanks