Further to my original message, I realised that my wife's laptop has a DP port so connected it to the 3rd monitor using DP > DP/DVI adapter > DVI cable. This worked fine.
In addition, I tried unplugging the 2nd monitor (HDMI>DVI) to see if it was to do with the load or amount of monitors. Nevertheless, the 3rd monitor keeps cycling through on and off.
... additional info, I have also noticed that when the screen flashes on, the mouse pointer freezes for a half second, works properly until the screen has switched off and back on again and then freezes again.
If there is no power to the monitor, the mouse pointer freezing still occurs and it only stops if I actually disconnect the DP/DVI adaptor/DVI cable.