No idea how I'd widdle it down to just 3, but just quickly scanning what's already posted, I'd have to say these would be candidates for most wanted (sooner rather than later anyway) in my case:
* The Sound of Music
* Raiders of the Lost Ark -- and probably also Last Crusade, especially since we'll likely end up w/ a boxset and/or simultaneous release anyway
* Alien and Aliens -- including both kinda like Raiders and Last Crusade ... sorta ... and assuming a worthy new HD transfer of course
Oh wait, oops, nobody's actually listed Alien or Aliens yet ...

* Jurassic Park -- well, maybe, but probably mostly because it'd make my son happy while I do like it as well
* Back to the Future -- I guess throw in the sequels also ... sorta ... since they (like w/ the other listed movie franchises) will inevitably be released simultaneously ... but if not, I wouldn't miss the sequels that much
* Lawrence of Arabia -- but partly because I'm limiting to the already listed titles and partly because I can't seem to locate my DVD of it anymore,
but certainly not because I can easily find time -- not(!) -- to sit down for such an epic length classic, and I do hate to only watch 1/2 of a film at a time ...
Hmmm... on 2nd thought, cancel this choice since I already have more than 3 anyway ... 
* Star Wars OT -- especially in their original non-revisionist versions, but w/ whatever film restorations and proper new HD transfer done; otherwise, I can wait since the special edition DVDs are actually quite good and hold up quite well on my 61" display
Some other honorable mentions (from already listed choices, after taking another look or two

):
* Saving Private Ryan -- maybe my memory is faulty, but I seem to remember thinking the DVD was still quite good; if not, this would probably make the main list
* Vertigo and Rear Window -- maybe these should actually go higher for me, but then again, I already couldn't widdle down to 3

* The Red Violin -- would probably put this lower *if* the existing DVD quality was actually all that good
* Pulp Fiction -- probably should actually go higher
* Few different Woody Allen flicks (though only Manhattan seems to have been listed so far) -- assuming they would actually look all that much better on BD of course

* Thief of Bagdad -- maybe should go higher because I never owned the DVD (and don't really want to spend the $$$ on the Criterion DVD at this point)
* Citizen Kane -- maybe ... since I haven't even gotten around to watching my DVD yet (due mainly to the WAF on top of the kids-taking-over-the-HT factor)

If I actually consider other choices (besides Alien and Aliens) not already posted by others, I'm sure there are more I could include -- and maybe some will bump off various titles I've included above, if I need to widdle down to 3 ...
_Man_
Edited by ManW_TheUncool - 9/28/09 at 9:29pm