No:
There are no 3-monitor cards in consumer-land (the Parhelia was an ill-fated attempt, but Windows only). There may be multi monitor cards for workstations fo rmegabucks but not in general commerce.
Yes:
If you install a second video card, your desktop can extend to as many extra monitors as you have card capabiliy for. So an additional 2-output card would give you 4 montior capacity.
The card has to be Mac comptaible.
Dont:
Confuse PCI-Express (PCI-e) and PCI-X they are different things entirely. There are no PCI-X video cards, so you would use a plain PCI video card in a PCI-X G5 tower
Determine which one your Mac uses and stick to that.