You can add any of the Series III consoles and rack mixers as monitor mixer and two units is not the limit. Important is that one of the mixers (FOH) is the "clock" of the network in order to make network presets (Stagebox Mode, Monitor Mixer Mode and Stand Alone Mode) function properly.
PreSonus has a lot of helpful information. There are two special manuals, which you can find in the download area of your mixer(s):
And there are several Technical Articles (partly with texts from the manuals):
A combination of two consoles and one rack mixer will need an AVB switch (SW5E). The NSB stageboxes have built-in AVB splitters.