This is a fork of moboot, the Mobile Open Bootloader originally made for the HP Touchpad, which xndcn made available for the HP Veer too.
Building moboot for the Pre 3 is easy.
Download this exact GCC distribution and place it somewhere in your filesystem.
Add the bin subdirectory of the extracted archive file to your $PATH, and then call make.
You'll find moboot's uImage ready to be membooted in build-pre3, it's called lk.uImage - memboot it through novacom:
$ novacom boot mem:// < lk.uImage
- for some strange reason, webOS likes to "disable" GPIO pins in a way that makes some of them unable to work even in moboot - make sure to boot into webOS and set your volume slider to "no sound" before using moboot, otherwise the menu will not work.