The ASUS BP1AD will only support 2 independent displays at a time. It is likely a HW or BIOS limitation of the system. See page 31 and 72-73 of the attached users manual.
A workaround would be add a Display Link based port extender.
Intel Core i7 4770 (3.40GHz)
- 8GB DDR3 1TB HDD Capacity
- Windows 7 Pro 64 bit / Windows 8 Pro 64 Bit
- Intel HD Graphics 4600
- Certificated by Energy Star 5.2
- Support 2 Independent Displays
That is not the specs on this computer, (16GB DDR3, i5 Processor in this) though I'm fairly certain it's not going to make a difference. Either way, that's pretty stupid on ASUS' part...take a system that is more than capable of doing what we need...advertise is as "Intel HD Graphics 4600" without specifying any limitations...and then cut one-third of it off?
Sounds like ASUS has a problem misleading their customers.