@D2 The 3-month rule itself was never explicitly stated or enforced anywhere. It just kind of arose as a code of conduct among scanlators back in the early days of scanlating. That and the wait period for picking up a series after another group drops it, before the first-come-first-served free-for-all. I might sound old, but those were times when groups had mutual respect for each other. Not something I see often with new groups considering all the snipes and crap that's plagued the industry in relatively recent times.
Thing is, any attempt to enforce a rule system would make the industry seem "official," potentially bringing the long arm of the copyright law down from the actual official sources. Keeping things unofficial and implied and just hoping that we don't start going at each other's throats is the safest way to go. People seem to be more selfish these days though, too much for any code of conduct to have any effect.
I agree though that going by the number of chapters over a fixed amount of time would be a better idea.