A couple of random thoughts that I had after reading more of your thread...
I understand that you're super into this couple, and that's a fantastic experience in itself already. I'm just wondering if you're aware of how many couples there are out there that would bend over backwards just to take you out on a date - did you know that you're called a "unicorn" in poly terminology? Just thought it might be worth mentioning, as so far it seems like both of your serious attractions have been to couples, and meeting a couple that has no authority over you, or responsibility (other than to treat you with love/respect, of course) over your life might be a more plausible set up to pursue relationship without societal encumbrances looming over your potential love. I definitely can see that now may not the time for this, as you've developed feelings for this particular couple, but I do think that it's worth mentioning as food for thought.
Another thought I had was: If the three of you end up pursuing a relationship with each other, and it grows serious, do you think that you would be willing to change schools to protect A's professional status? I totally blanked on what km pointed out: He's already married, so said form would probably not do a lot of good. Sorry for not putting 2 and 2 together in regards to that/giving you false hope.
This is kind of putting the cart before the horse, but it might be worth giving some thought to.
I can totally understand the fear of rejection, but I also think that there is a lot of power in having all the information, and the only way to have that information is to be honest and communicate your feelings to them.