Following in the footsteps of the CyanogenMod 7 alpha build that recently became available on the HTC EVO 3D last week, it looks like MIUI version 1.9.16, alpha 3, is ready to be downloaded and flashed – but only if you are already participating in the closed beta. This is because this most recent update should only be flashed over the base ROM, which I have not been able to find for public download.
According to the developers who are working on this, it is 98% ready to be used as a daily driver, although Bluetooth is not working yet. In addition, there is a bug that causes the EVO 3D to reboot if the volume is adjusted during a call.
The above video actually demos version 1.9.2 on the HTC EVO 4G, but this should give you a good idea of what to expect when it does become available on the EVO 3D.
MIUI is quite a bit different than most other custom ROMs, so there might be a slight learning curve at the beginning. Therefore, since it seems that a release candidate is almost ready, it might be a good idea to check out this helpful guide for beginners.
Update: As it turns out, MIUI is no longer in a closed beta, and the ZIP file in the link below can be flashed to a rooted HTC EVO 3D. Enjoy![MIUI Forums]