I read on a thread that you can't have three way wars, meaning if you have a pair of wars (1) France v. Sweden and (2) Russia v. Sweden, that France and Russian cannot dow the other.
In my game, with this situation, there was a third war (3) Spain v. Russia. Spain and France allied and the very next turn Russia was attacking France!! The other wars were still going on.
I mentioned in the beta feedback section that I had gotten into a three-way war, but there was no response. So, yeah, it is still possible under some circumstances.
Can you provide any more information as to how the DOW happened -- was it over a protectorate or a treaty obligation? Looking briefly at the code, it doesn't seem it could have been a standard DOW.