You can install all 3, no problem there.
Startx uses, among other files, the ~/.xinitrc file and as you might have noticed all three WM's you mention have a line in the .xinitrc file to make sure that specific WM is started. Putting all three in the same .xinitrc file won't work. One solution to this is to create 3 WM specific .xinitrc files and link the one you want to use to the real .xinitrc.
holds all the metacity specific entries,
the fluxbox specific entries,
the sawfish specific entries.
If those are in place you can link the one you want to use to ~/.xinitrc:
ln -s ~/.xinitrc.fluxbox ~/.xinitrc
If you want to switch to say metacity:
ln -s ~/.xinitrc.metacity ~/.xinitrc
Hope this helps.