HIS Radeon HD 6790 will not detect 3rd Monitor

As per my guide
http://www.tomshardware.com/forum/344343-33-eyefinity-setup-guide

you should connect you monitors in the following way.

Video Card - DVI to DVI of Monitor (S24B300)
Video Card - DVI to DVI of Monitor (S25B300)
Video Card - Display Port to Active Adapter to DVI of Monitor (HP 2511)

as a test you can also test all the monitors individually on each of their connections to check its all working before connecting all 3 at once. its possible you could have a faulty cable, active adatper, port on video card or monitor.

any of those will cause the problem you are seeing.

All the cables are good all 3 monitors are good the adapter works as long as one of the other monitors are not detected.
I read your guide and every thing I could find on google before I made this post. Thanks for your reply.

Two things you might want to try.

1) Use Driver Sweeper software to remove AMD drivers entirely, then reinstall (having monitors connected.

2) Look for your monitor driver and install it manually (usually a file you right click and select install)

EDIT: Have you tried hooking just the un-detected monitor? What was result?