While 3) does get somewhat closer to violating some spirit of free speech, I personally don't think it really gets to the point of being a violation of it because, as you said, it's an application of C's free speech andit just gives D, E and F the same circumstances that C has in 2) through informing them of the circumstances with B and A. In intent, C might be trying to stop B from having this specific platform, but I feel that the spirit of free speech doesn't really entitle B to a platform to begin with, so at least in that this isn't a violation in the realm of concrete actions separated from intent. However, money is something that is required to survive pretty much globally and A's choice to exercise their free speech one way or another, depending on how successful the letter writing campaign is, is now influenced by whether or not they want to eat tomorrow, which taints the actual freedom of that choice... but any notable financial coercion would, in addition to C, be equally, if not more, on the shoulders of D, E, and F... but they're also free to express their free speech by not supporting a platform and the notion that they have to support A or be free speech hating authoritarian leftists is also a violation of the spirit of free speech...
...shit's complicated and concepts like this are wide. I don't even know if I have a coherent definition of the social concept of free speech as opposed to just vague feelings about it. But I guess that's also liberating, to be able to confidently say "I don't know" and own one's feelings of uncertainty as one's own as opposed to trying to force oneself to think on either side just to not feel the insecurity of mental emptiness?