Please attach your xorg log, xrandr --verbose output, and dmesg output.  Also,
are two of the monitors identical?  Tonga only has two independent non-DP clock
sources so it can only driver two independent sets of non-DP timing.  Since you
are using 3 non-DP displays, at least two of the displays need to be using the
same mode.  You may have to manually set the mode on the 3rd display using
xrandr to make it match the mode selected on one of the other displays.