An HD 6670 without DP output port can only support 2 monitors indeed. I'd suggest getting another cheap AMD card if your motherboard has a second slot - something like HD 5450 will do. http://www.newegg.com/Product/Product.aspx?Item=N82E16814131339
Though, if you choose to buy a different card, make sure it is an AMD card. You would be calling trouble by trying to run both AMD and Nvidia card at the same time on one PC.