Adding a third 4890 will be an incremental upgrade. At best it will provide a 40% increase, though it seems on average the improvement of triple, vs dual, crossfire will be less than that. Still, it's seems to be a significant improvement. If your board can do a 16x/8x/8x PCI-E configuration, it's not too expensive for you, and you just want that extra bit of speed, then sure it may be worth it for you. If either of those criteria is false, then you may want to save up for two 5870s or two 5970s.