all three are now out on iTunes
for free, but you can still spend money on them:
If you’re familiar with the iOS incarnations of the first two games, you’ll pretty much know what to expect with Infinity. You get the complete game in standard definition for free, which still looks pretty darn good after all these years, but alternately for $1.99 you can download the high resolution texture pack to take things up a notch. Then there’s a Master Chief mode for 99˘ which will give you tons of cheat options and a level select, an another 99˘ IAP for a variety of different crosshairs.
I never really got to play any of them, so I will be grabbing the free versions to start and maybe upgrading to the hi res versions.