There are many open ended questions here.
I suggest that you read a couple of the documents in our Resources Library (Tech notes in particular).
Tech notes are here: http://www.qlik.com/us/explore/resources/technical-briefs?language=english
Two comments though:
1. 3 TB is a lot of data, but that does necessarily translate to 3 TB in memory QlikView data as QlikView uses highly efficient algorithms to compress data (storing distinct values only).
2. Every application in QlikView should cater a specific need. In very few cases is the full data needed.
Hampus von Post