Oh, I think you misunderstood me. I meant you should probably order the 3 TB with the computer, so you'll have the 3 TB when you get it. That's probably the best route for most consumers.
"most consumers" shouldn't be buying a Mac Pro in the first place. It is total overkill and they will never use the expandability. People that actually use Mac Pros will already have HDs to throw in there or will order them from newegg or wherever when they buy the Mac Pro from Apple. The mentality of paying (a lot) more just so that it ships from apple with it installed is ridiculous for people that need Mac Pros.
If you order from Newegg, you'll get your HDs on or before the day that you'd get the Mac Pro (using standard shipping).