Loren Pechtel
Explorer
Given that you can very easily make a Flaming Shocking Icing Longsword, I have no problem allowing a Flaming Flaming Flaming Longsword.
I'm not sure it's 100% by the rules, but it certainly isn't unbalanced (in most cases, it's weaker than the FSIL).
It's weaker if you treat it as 1d6 fire + 1d6 fire + 1d6 fire. If you let it do 3d6 fire it's stronger because it's a lot more likely to get through the resistance.
Personally, I wouldn't allow a flaming/icing weapon anyway--how can a sword be both hot and cold at the same time? (You could make one but you couldn't have both turned on at once.) This isn't a balance issue, it just feels wrong.