Re: 3 Monitor setup
Most video cards that have multiple ports only allow 2 connections out. I have an AMD Radeon EyeFinity card and it needs a display port adapter for the third connection to work correctly.
You can have 2 different video cards in. Preferably they be the same type, since you have nvidia, get another nvidia. Since you said you put an older video card in, it may not be supported by your current OS if it's a legacy video card. For a third monitor you don't really need a powerful video card. You can get by with a basic nvidia card.