Others here note it's really "3.5% if there's no one seriously opposing their objectives" but in my opinion that's a meaningless rule. Of course in those cases non-conflict resolves the issue.
Those 3.5% are encouraging for all social movements, who suffer (and/or have friends/family who suffer) from some issue in the system, have perhaps developed a good plan out of it, but think they are too small to make a difference.
From that perspective it becomes clearer what a 3.5% rule is getting at - 3.5% of the population mobilised is enough to overwhelm any ruling class that isn't on top of its game, especially if mass shooting of people is still of the table or if the 3.5% includes a lot of people from the upper classes. It isn't about whether an issue is supported by 3.5% of the population or more, it is a question of whether that fraction of society is actively trying to topple a government system.