If you're only going to get two of the three, then I'd say Fallout 3 and Valkyria.
Fallout 3 is GOTY material. It is an absolutely engrossing RPG that you can sink 50 hours in, then happily restart and sink another 50 playing the same game a different way. I have played this more than any other game on my PS3.
VC is a solid title and beautifully executed. Now, I'm not personally a huge fan of this one because I've just never been able to wrap my head around the "girl with the pom-pom hair and the mini-skirt firing an AK-47" JRPG character design thing going on here, but it is a good game if you can suspend your disbelief (you'll need a crane).
R2 is the one that I think is least pressing to own, because it'll likely go to a $29.99 greatest hits repackage ahead of the other two, and it'll be obsoleted in two months by Killzone 2.
