You say you have the 780, but there are 4 versions of that model, which you don't specify.
According to the spec, there is:
MT which has 2xPCI, 1xPCI-e16, 1xPCI-e1 and supports full height card. Not sure on card length though and support for dual-slot cards.
DT which has 2xPCI, 1xPCI-e16 and supports full height card. Not sure on card length though and support for dual-slot cards.
SFF which has 1xPCI, 1xPCI-e16 and supports half height card. Not sure on card length though and support for dual-slot cards.
USFF which has no expansion.
The guide book online suggest RADEON HD 3450/3470 or Nvidia 9300GE none of which are three output cards.
According to Nvidia a card like
this should support 3 outputs at the same time, and should fit in the case, but it is not very powerful for games.
Again, something like
this should suport 3 displays, but it might be a tighter fit in the case.
This supports 4, but again might be too big.
Only the GTX730 will fit in the SFF version but not sure if you can use the third port like this.
To work out whether a card will fit, you will have to take the side off the case, and measure the space available around the PCI-e slot, and the length between the back plate and HDD racks or the front of the case.
