Unfortunately I do not think it good news for you (if you are the Landlord) Ignorance of the scheme is an excuse I'm afraid
The 3x the deposit figure is the set penalty so is not a figure picked out of thin air. I also do not think it is relevant that the tenant has the deposit back, but I am not sure about the joint tenancy and one claiming or about timescales
Have a look at the DirectGov website
http://www.direct.gov.uk/en/TenancyDeposit/DG_ 066380 as that has good info on it
or Landlord Zone is quite good.
http://www.landlordzone.co.uk/