PT2 hasn't been on sale for quite a while now, so you couldn't have registered it last month? If you registered it before that and wish to buy PT3 you will get a discount from the cost of PT3 - you should be able to link your PT2 purchase in My Account -> My Orders but if not then please create a Support ticket and someone will help you out.
You can check your PT3 code via My Account > Registration Codes - if you can't get it to work please create a ticket as I suggested and someone will help you out.