The ACR versions are dedicated to Photoshop versions. The MkIII is supported by ACR 4.x, which required CS3 or Lightroom whatever.
If you want to wait for CS4 (which is allegedly not far away), then convert the files in DNG format; that will be accepted by ACR 3.x as well.
Adobe's DNG converter is free. You need the newest version, which supports your camera. You can't download it alone; it comes only in a package with ACR. Download the package and throw away ACR 4.4.
The handling is easy, the process is fast. However, I recommend to archive the original raw file, not the DNG.
When working with ACR, the adjustments can be saved either in sidecar files (XMP) or in the ACR database; stay clear of the latter (I have not worked with ACR 3, but I guess this was the way earlier as well).