cluster-ssh with multiple monitors

not sure about how/if this fits together with the XML file, but you should be able to use a .csshrc file in your home directory with a mix of some configuration options to accomplish what you need. From the cssh(1) man page I found at this site:

screen_reserve_top = 0
screen_reserve_bottom = 60
screen_reserve_left = 0
screen_reserve_right = 0
Number of pixels from the screen side to reserve when calculating screen geometry for tiling. 
Setting this to something like 50 will help keep cssh from positioning windows over your window 
manager's menu bar if it draws one at that side of the screen.

so, if you set "screen_reserve_right" to the combined width of your left two monitors, the terminals should be limited to your right-most monitor. If you use "screen_reserve_left" and "screen_reserve_right" you should be able to relegate it to your center monitor, and "screen_reserve_left" should allow you to relegate it to your left-most monitor. Apologies if you've been through that already, but figured it may be worth a shot.

There are a few things to check. Some are listed here.

Does your window-manager support multi-screen.

They all work on multi-screen, but not all support multi-screen features, such as maximise to just the current screen. (I could not find out if Openbox has this support).

Some programs will insist on setting their own location, and geometry.

X11 has several levels of insistence that an application can use. The window-manager can choose to honour at any level (all, some, none). Some window-managers are configurable, some are configurable per app (because most apps suggest good values, but some don't).

An example

I use KDE Plasma. I have it configured the way I like. It works for most apps. I then have set overrides for the few apps that need it:

• I have different title-bar colours for some apps.
• Ignore requested geometry for another (I suspect this may be of use).
• Open on desktop x for another. Open on screen y for another.
• Minimum size override for web-browser (so I can make it smaller, and test websites on small screen easily).
• Focus stealing prevention, on another obnoxious app that keeps grabbing keyboard focus.
• etc.