It is a very glad news to know Retevis RT3S supports flashing open source firmware OpenGD77. This need to thanks VK3KYY Roger, Colin G3EML,Daniel F1RMB OPEN GD77 team memebers.
But this firmware is an Alpha version now, meaning not all functionality from the OpenGD77 is included and that the firmware will have many bugs.
The Firmware installation instructions here:
and video instructions here: