Was boat B called as a witness to the hearing? If boat A thinks she fouled another boat she should take a penalty no matter if the other hails protest or not. If boat B does not think she was fouled (thus no protest call at the time) that should have come out at the hearing. It sounds like boat C tries to win in the room what she can't win on the water. Which boat are you?  :-)

Gulboy
Firstly normally a boat that breaks a rule of part2 needs to take 2 turns unless changed in the SIs. 
Did A on port tack keep clear of B and or C ?

Assuming one turn was in the SIs, the argument that the penalty turn was for fouling B and not C I think does not fly.  If both starboard boats had to avoid, I think it counts as just one incident.  

Charles
If there were 2 breaches then 2 Penalties are required. 
However if you break a rule of part 2 and hit a mark during the same incident you don’t have to take an extra turn
RRS 41
a)when a boat may have broken a rule of Part 2 and rule 31 in the same incident she need not take the penalty for breaking rule 31;

I think if B and C were well separated, two opportunities for fouls so possibly two penalties.  If they were close so that (assuming both were fouled) having fouled one, A could not avoid fouling the other, I take that as one incident.

It has not been established the boat B thought they were fouled. For a jury to DSQ boat A based on boat C's testimony alone is very odd. 

Randy
I’d be surprised if the PC would not hear from both A and B as well as A and make a decision based on all the evidence. 

The whole question is whether this is one incident or two.  In the US, we have Appeal 65 which, while not defining what an incident is, says that two occurrences are one incident if the second occurrence was the 'inevitable result' of the first.

Another key is that a boat has to know she is being protested.  As there was only one hail and flag it is not unreasonable for Boat A to assume that there is only one protest.  If Boat C thinks it is two incidents then she should have hailed protest twice and filed two protests.  Otherwise, you only have to take one penalty per incident no matter how many rules have been broken in that incident.

I agree, that's why I asked the OP. There is so little information in the post. It smacks of sea lawyering. Why would boat C assume that boat B was fouled if boat B made no protest at the time of the incident? After seeing boat A take her penalty in response to *your* protest hail, why would you take it to the room anyway? I've RO'd fleets were many races and series were won in the room rather than on the water. So I'm suspicious about the wording and implications in the OP.

If the PC thinks that it is two incidents and wants to protest Boat A for the A/B incident she could try to act under rule 60.3(a) but this has problems as the report was not from a representative of the boat herself.  The validity of that protest would be on shaky ground, IMHO.

What exactly is meant by "marginally crosses"?

Absent a reasonable apprehension of collision and a course change by Starboard, "marginally" is extraneous. If A didn't observe a course change from either starboard boat she's within her rights to keep sailing, red flag(s) or no. Penalty turn (can we assume the SIs reduced RRS 44 from two turns to one?) was worthwhile insurance.

I'd tend to agree, if C intended to lodge two protests for what she perceived as two separate incidents she needed to inform the protestee of both.