With the release of TWRP 3.2.3-0 update, now official TWRP support is available for Xiaomi Mi Max 3 and HTC U12+. However, the Mi Max 3 is an old month smartphone its kernel source code is not available yet.
As reported by XDA Developers, the kernel source code for the HTC U12+ is available so developers can start building latest custom ROMs, especially for the Android 9.0 Pie.
The development for Mi MAX may be delayed due to non-availability of kernel source code, while the problem with the HTC U12+ is its popularity. There are many developers who’ve bought the device yet.
Download TWRP for Mi Max 3 and HTC U12+ from below link: