|You can't. When you install an add-on GPU the on-board one is automatically disabled to avoid driver & resource conflict. So even if you found a way to stop the on-board GPU being disabled, it wouldn't work because of conflicts with the add-on GPU.|
You can, however, buy an add-on GPU which itself supports up to three monitors.
Pricey though. I think there are several in the GeForce series, can't remember the model number.