Ok, I voted Assassin's Creed before fully reading your post.
But, I'll say buy them in this order
GT5, NFS:HP, AC:B
Why? Because GT5 probably won't drop in price for over a year. AC:B you can probably get new for $20 6 months later (that is how I bought ACII), and NFS:HP you can probably get at a decent discount new 3 months later. I only put it this way because you want all 3 so you might as well get the best bang for the buck.








