In case there were more people with issues getting PBP3 working with MPLABX I have some more information. Presuming you have Already installed the toolchain plugin into MPLABX, then you need to also goto TOOLS then OPTIONS then EMBEDDED then ADD. You should see the PBP compiler listed. Add it, then make it your default compiler. Presto! Make a new project and you should now be able to select PBP3 properly.