I use to keep the USB cable connected, press and hold all three hardware buttons until Rk Tools show the message "Found One LOADER Device" at the bottom - like on this pic:
Don't worry if the screen on your device seems stuck, bootloader has nothing to display.
[edit]
This triangle you mentioned is Boyue's locked down recovery mode. Next+power is how you'll enter TWRP recovery after its installation.
Bootloader mode requires prev+next+power to be pressed simultaneously at power on.