There are very few motherboards that support using both the motherboard integrated graphics unit at the same time as a dedicated graphics card. Some newer motherboards support Hybrid Crossfire or Hybrid SLI to boost the performance of the dedicated card using the onboard graphics, which to me would suggest that they are also capable of being used independantly.
It may be that you simply need to go into the BIOS and enable some option that switches the integrated graphics back on irrespective of the dedicated graphics, but I wouldn't bet on it being there or working the way you expect. Information on which systems support using the integrated at the same time as dedicated graphics is patchy at best and depends entirely on the manufacturer.
The other way to get that third monitor would be to use a cheap PCI graphics card in a spare slot. You could probably get one that would work for £30.
Another question about the same thing, and the long and short of it is that it depends very much on your motherboard and whether it supports re-enabling the integrated graphics. It works for some boards but not others, not all motherboards are created equal, YMMV.