I’m puzzled by some of the advice being given here. “Don’t try any more home remedies” The OP hasn’t been trying ‘home remedies’ she merely bathed the eyes daily to clear any debris (and by doing so has possibly prevented a secondary infection).
“You should take them to the vet.” Why? The infection was obviously self limiting – in other words it ‘ran its course’ and went away. The OP says the pig’s eyes are no longer inflamed and so what exactly is the vet going to do now?
“It can be a sign of a UTI” The OP did not mention the cavies had any signs of ill health other than a conjunctivitis type infection. True, a UTI is often accompanied by runny eyes but this does not seem to be the case here.
The OP was asking whether this type of condition was linked to lack of vitamin C. The answer is no (although of course, a cavy in good health is more likely to be able to clear itself of infections more quickly).
The problem is obviously contagious as the second cavy caught it. The most common cause of conjunctivitis in guinea pigs is Chlamydia caviae (sometimes called ‘chlamydial conjunctivitis’ or ‘occular chlamydia’ ) and I am very surprised that it’s not mentioned anywhere on the Guinealynx medical reference pages. It is often the cause of mild eye infections exactly like the one that the OP described.
The cavy Chalmydia is ‘self limiting’ but the reason that breeders will often seek veterinary treatment is because it's very contagious. It can spread through a whole breeding colony of cavies and can occasionally cause abortion in pregnant sows. This isn’t the situation in the OP’s small group of boars.
It’s possible that the third boar will also get it, or it might be that he has already had it and is immune to it (it’s rare for a healthy cavy to get it a second time). Just out of interest, is this third cavy the newest addition to the group – perhaps he or his previous cage mates had it and it was him who brought it in?
The most important thing is for the three cavies to have no contact with any other cavies for at least a few weeks – no ‘play dates’ or handling other owners cavies. By the way, Chlamydia caviae is a cavy specific infection, it is NOT the same as human Chlamydia (Chlamydia trachomatis) or the parrot disease, psittacosis (Chlamydia psittaci) so no-one will catch anything from the cavies !