You need to be more specific on the motherboard make and model number. There is no way to know without know which board you have.
EDIT: if it is this board (http://www.newegg.com/Product/Product.aspx?Item=N82E16813128594) then yes, you can do 3-way SLI but the cards will be running x8, x4, x4. You do not need a PLX chip. You only need one if you want the cards to run with more PCIe lanes such as x16, x16 or x16, x8, x8.