No... It's the equivalent of 3 4850s. It's like running 1 4870x2 + 1 4870 = 3 4870s
A 4850x2 is essentially 2 4850s, it's not like a gtx 295 (which isnt 2 graphic cards of 1 chip type ontop of each other). So adding another one is like have 3 4850s, because it's having 3 cards of the same chip type.
Overshock, you are thinking of when you run 1 4890 and one 4870. They are the same chip set so they work in cross fire, but the 4870 downclocks its self to the level of the 4850 to make it work, or if you run a 4870 1 gig and a 4870 512, the lower card cant make its self better and run like the better card, but the better can lower its self then the same level as the lower card. Of course though 1 4870 and 1 4850 is better then a 1 4870, it'll just be a waist of your money.
For the original poster's question, 3 4850s would be useless (no offense), because most games dont scale well for 3-4 way SLi and as a previous poster already said new cards are coming out, though if you want to move to 2 4850s from 1 4850, that would be a good move because 1 4850 can be found as low as 100 USD and would boost your performance substantially for the time being untill you decide to upgrade again.