If one is dead, the others will probably die soon. If you have any other sort of marine battery you could replace the bad one on a temporary basis, and run it until the others start giving you trouble. I wouldn't buy a new $250 battery and put it in the string with 2 that are old. The old ones will drag the new one down before it's time, and you will constantly be replacing batteries.

If I didn't have a lot of funds, I would consider buying 3 group 31 batteries from somewhere like Advance Auto. Use their discounts, and you can probably get them for $300. I've had good luck with them and use them on my boat.