What does 3Q8 mean in the aircraft model Boeing 767-3Q8?
Those are Boeing's customer codes. They denote which airline the aircraft was originally built for, not necessarily the airline that owns/operates the aircraft currently. For example, Southwest's customer code is H4 but they operate some 737-7BD's, which are from the AirTran acquisition. In your case, the Q8 means it was originally built for ILFC, a leasing corporation.  A full list is available here: 
http://en.wikipedia.org/wiki/List_of_Boeing_customer_codes

Your assumption about replacing the customer code with 0s to get the plain old model is correct.