Would all of the sentences below be used?
(a) The time will come when you regret having said that.
(b) The time will come when you will regret having said that.
(c) The time will come when you would regret having said that.
If so, are there any difference in meaning between each of them?
Well, I certainly wouldn't hesitate to use it without the second 'will'. As with most cases, omitting a word makes it slightly less formal and it may be better not to use it in an English exam. There is certainly no difference in meaning between the two.
Thinking about those two gives me a headache. Hmm, ... I think if it was "A time ..." instead of "The time ..." it would be a bit more natural. I still wouldn't recommend them much.