Android 4.3 is the latest Android OS and now only available for Nexus 7 , Galaxy S4, Galaxy Note 3 and some others latest Android smartphones. We know that Samsung still not yet official releases Android 4.3 update for Galaxy S3 and Note 2. According from some online info has mentioned that Android 4.3 update is coming soon to Samsung Galaxy S3 and Note before end of this year.
Here has another Android 4.3 Custom ROM for T Mobile Galaxy S3 SGH-T999. This Rom firmware version is T999UVUEMJ4 and only is a test version. Since this is a leaked ROM, expect some bugs. But, if you are developer and want to start test this Android 4.3 ROM, the you can download the ROM from the link as below:
Note: Flashing or Modified Galaxy S3 firmware will void the phone warranty. Refer here (Link) for more detail before start flash the Rom.
T-Mobile Galaxy S3 SGH-T999 Android 4.3 ROM file → Download link
SGH-T999 Android 4.3 Modem file → Download link