What motherboard do you have? Do you already have both 4870s? You can use a third GPU to support additional monitors, but you need to make sure that your motherboard can accommodate 3 GPU.
If your motherboard cant, you can switch it for 2 5770s in CF or a 5870, they will perform about the same but will allow you to use 3 monitors, but one of them will need to be display port or you will need an active adapter.
I have a Gigabyte that has 2 PCIe 2.0 x16 slot, one PCIe 2.0 x8 slot. I have the 4870s two running at 16 (currently) but with 3 they would all run at 8x.