How to extend from 2 to 3 X workspace in UdooX86 Ultra with 3 monitors connected ?

For my new SBC UdooX86 Ultra I have successfully installed FreeBSD 12.0 (version 14th August 2017) and rebuilding the kernel with drm-next from GIT clone I have also obtained a xfce4 desktop (thanks for this howto to Benjamin user in UdooX86 mailing list).

Now I have this problem: the UdooX86 have three output video: 1 HDMI and 2 DP.
I have successfully obtained the FullHD video signal with a "hot" connection of the third monitor on the second DP port but the xfce4 display configurator or arandr either recognize the third monitor but assign only 2 workspace so actually for me it's impossible to have 3 differents workspace. Infact, when I put the third monitor outside the available workspace, the monitor itself lose the signal and become black. But the UdooX86 don't freeze and it's possibile to reboot from another TTY so I think that it's a software and not an hardware problem.
Slackware 14.2 it's able to recognize the signal of either 3 monitors from the boot.
Ubuntu 17.04 and Debian 9.1 have and hardware freeze connecting hot the third monitor.

Now I ask you if with FreeBSD it's possible to assign to arandr 3 different workspace indeed of 2.

Thanks in advance

UPDATE: 20th August 2017
I add the screenshot of error by arandr
 

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  • Screenshot_2017-08-20_20-37-29.jpg
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You should use $ xrandr to check for detected and active outputs. In my experience, if xrandr doesn't list an output, there is a hardware or driver problem. Some graphics chipsets only supports a dual display setup (in other words, only two monitors active at the same time) even if they have three outputs. YMMV.
 
You should use $ xrandr to check for detected and active outputs. In my experience, if xrandr doesn't list an output, there is a hardware or driver problem. Some graphics chipsets only supports a dual display setup (in other words, only two monitors active at the same time) even if they have three outputs. YMMV.

Dear tingo

this is FreeBSD xrandr output.
Have been linked 1 HDMI and 2 DP cables but xrandr see them as 3 HDMI.
The chipset support 3 monitor: Intel® HD Graphics 405 up to 700 MHz 16 execution units (ULTRA version)
 

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  • FreeBSD-xrandr_output_with_3_monitors-20180820.txt
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And do you have picture on all three monitors now?
If the answer is "yes", try to arrange the displays the way you want with the --left-of or --right-of arguments to xrandr.
 
And do you have picture on all three monitors now?
If the answer is "yes", try to arrange the displays the way you want with the --left-of or --right-of arguments to xrandr.

Yes I have the picture on all three monitors but the problem is that 1 of the three monitor is a mirror of 1 of the other two and indeed I want 3 different workspace.
 
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