How to get a third dependent drop down list to work?

Hello,

I am trying to work out how to make 3 drop down lists work together. I have got it so the first two drop-downs work, but cant work out how to get the third to work with the other two.

Im trying to work out printing prices depending on wether the paper used is the Shops own paper or Customers paper, the print type required, and finally the paper size (A4 or A3).

Can anyone have a look at my spreadsheet, there are two sheets, one called Selection, the other called Prices. The three choices are made on the selection sheet.

Cheers

Tom