CyanogenMod is one of the most popular Android ROMs and is supported on a ton of devices already. The CyanogenMod 10 version being offered for the MetroPCS SGS3 is a nightly build, meaning it is the experimental product of a day's work. Bugs are to be expected, but CM10 is getting quite stable overall. There are no monthly builds for this device, but that might happen soon. The monthly "M Snapshots" tend to be a little more sturdy.
If your Galaxy S III on MetroPCS is unlocked and ready for action, hit up the source below to grab the ROM. It's 162MB in size.