If A forced B to have sex with C, but C doesn’t know B is being forced is C raping B? Is A raping B? What is A doing to C?
Is C having sex against their will, or is it voluntary on their part?
C is voluntarely having sex with B, but C doesn’t know B is being forced.
Interestingly, the various statutes I’ve skimmed say it’s rape if a person is forced into sex, though none specify that the sex need be the one doing the forcing. A is committing rape, while C can try to argue his participation in the crime is unintentional. I suppose a possible case would be a pimp who orders his hookers, under threat of going upside their heads, to turn tricks. I don’t think a successful rape prosecution of a pimp has ever been done under these conditions, though.
A is the rapist. It’s like some really huge guy picking me up and throwing me at you, pushing you off a cliff to your death. Who’s the murderer ? The big guy; I was just the weapon.
I don’t know, but is it
Who’s raping whom?
Perhaps a little off the topic, but from a prosecutor’s point of view I would not push the rape issue at all. B and C had sex. C did it willingly, B under duress. So Let’s look at the situation around B. B was “Forced” into it by the original post. How? At gunpoint? Blackmail? It seems to me it would be much easier to make a case against A for any number of charges involving said coercion than to try to prove the rape thing.
One circumstance this could happen in seems fairly obvious to me. First, because I have trouble keeping things straight when using single-letter variables (especially when their pronunciation rhymes like b and c); I will be calling them Arnold, Beth, and Charlie. 
Charlie and Arnold are old friends, even though Charlie disapproves of Arnold’s questionable nature. Arnold knows Charlie has a thing for Beth, but she isn’t interested in him. So Arnold decides to “help” Charlie. One night all three happen to be at a club. Arnold slips a drug into Beth’s drink, and after making sure she drinks it, brings a -very- drunk Charlie over. He thinks Beth’s odd behaviour is because she’s also very drunk, and when she seems to respond to his advances, is delighted. Arnold drives the pair to Charlie’s place and…
Arnold has effectively raped Beth and Charlie’s just the poor sap caught in the middle, IMO.
Just as an aside, I find it remarkable that you knew of a situation that mirrored the OP’ers scenario * and it involved three adults whose names all began with A, B and C*.
I mean, what are the odds ? 
Rape aside, what is the case law for someone who is “compelled” to commit a crime? Forced at hidden gunpoint to participate in a robbery, drive a getaway car in a robbery so their child/spouse/parent/whomever is not killed, and so on.
The rape angle is particularly harsh because A forced B by compelling them. B then had to perform sexually with C in order to fulfill the demands of A. That means that even though B was being compelled, B also was sexually aroused enough to perform and complete the sex act to the point that would satisfy the demands of A.
I dunno. I am feeling like B is culpable. It was his body, he used it at the demand/behest of another.
Spin the genders, and we have A either female or male but B being female. She seduces C because she is compelled by A to do so. Same problem. She used her body to perform sexually because she was compelled to do so. She bears some responsibility for the rape act that she performed, although I suspect a jury would have trouble- because of gender bias- seeing a woman as a rapist in this scenario, whereas if B is male, they’d be able to see that more easily.
I’m not saying that’s right or proper, but I could see it happening. My take, no matter what, is that B is culpable but A forced B’s hand. There is a thread about the Manson killers and one who is up for parole, and it touches upon this very issue. Some claim that a lot of the Manson Family killers were brainwashed by Manson so severely that they were acting in a state of diminished capacity. Could the same argument be made in this A-B-C scenario? That due to duress from A, B ( of either gender ) felt compelled against his/her better judgement to perform sexually with C?
I cannot see C being charged with a crime.
Cartooniverse
I didn’t know of a situation. I said that a scenario along the lines of the OP’s outline was easy to imagine.
Oooh, just for the hell of it.
Suppose for the moment that C is a minor.
Oooh! And an illegal immigrant!
Does it matter if C is kinda dumb, or if C is sharp?
H3Knuckles, that was my attempt at wit. Apologies, as it apparently failed miserably. Your analogy did make perfect sense, honest.
Bricker? You need an agent AND a manager with puns like that. 
Would it help if it was a C sharp minor in concert with others?
What if they were all on a treadmill? Would they ever take off?
A favour
Couldn’t resist.