Hello there i found out the radeon manpages isn't right in DRI section:

Option "DRI" "integer"
              Define the maximum level of DRI to enable. Valid values are 2 for
DRI2 or 3 for DRI3.  The default is 2 for DRI2.

Following that i couln't enable DRI3, cat .local/share/xorg/Xorg.0.log | grep
-i dri3  will outputs:  RADEON(0): DRI3 disabled

But if one sets: Option "DRI3" "1"  then cat .local/share/xorg/Xorg.0.log |
grep -i dri3  will outputs: RADEON(0): DRI3 enabled

So i think you should edit the manpages like so:

Option "DRI3" "1"
              To enable DRI3 support...

Thanks to all of you