With X1900 you can only output 6 Screens in a resolution of 800 x 600
So if you select 3072x768 on the first TH2G the other TH2G won´t be able to output a triple screen because it exceeds the "canvas" of the graphics card.

A good help when having problems with TH2Gs is to disconnect the MacPro from the mains and not only put it in shutdown for a reset - that helped me in situations where I thought the TH2G was gone for good...

It also is a good idea to do the setuo with 2 TH2Gs with a second computer via remote desktop or screensharing... - makes life so much easier...