I'm currently running 3 displays on a Sapphire 5830, and I have read somewhere that at least one of the displays must be connected via a DisplayPort Cable. If you aren't using the DisplayPort connection on one of your screens, that could be your problem.
Hi w_marthe, tnx for the comment
i did use a DP-DVI after i read a topic here abt it and replaced it for my HDMI to DVI.
The software detecting 3 screens, but when i extend to my third screen it allows me only when i disable one of the other 2, so at the end no third screen ...