2&3 are the famous biggies, yes, but I would hate to be without a copy of
the fourth... after I heard the amazing recording of it by Michaelangeli/
Gracis (on EMI coupled, coupled to an equally legendary Ravel Piano
Concerto).  For Ashkenazy, I understand that 2&3 are his best, so I would
recommend just getting those, then furthering your Rachmaninoff concerto
cycle with other recordings, such as the Michaelangeli.  And if you do feel
like going on to the 1st, I'm sure there's an excellent, more
authoritative, single version of that as well.

This may take more time... may cost a little more...  but more fun
searching them out, you'll have the best of each piece, and you won't get
saddled down with pieces that you don't want (The economy of boxed sets is
kinda lost if you don't listen to half the recordings in the box!...  The
complete Mozart Edition??  AUUUUUUGH!!!!)

But DO make sure you've got the Michaelangeli Rach4/Ravel... Perhaps the
Ravel has been surpassed by the new Zimmerman/Boulez (IMNSHO, it HAS...
WOW!!!), but heck, ever since I heard M's Rach4, it's been my favorite of
the Rach concertos!

Albie