I haven't recorded the third message yet, but I suspect you'll get a lot of 'ones'

It's something that we should try though, fo' sure.
Other comments:
There's a lot of ones.
Spacing is definitely 'off', but this might not be relevant. It might well be, though.
Number Stations use a lot of 'fluff' and padding. If the only time they transmit were to be when it was an important message, that itself would let others know that a message was being broadcast and that its contents made sense. As a result, most of what they transmit is garbage. I suspect that this may be the case here. Some of what we here may well be padding.
A number station would rely on the listener having three things: The frequency, the timeslot that a message for them is going to be broadcast in and a one-time pad. Essentially, they are uncrackable and - unlike a coded email or message left on a website - cannot be traced to the receiver, so are still relevant in modern days. Safe to say that FD aren't going to need us to have a one-time pad. Instead they fostered us with a clue of '5C', which I'm sure rings a very loud bell with us all, while not being immediately helpful.
I suspect it is still a substitution puzzle, using some other data as a key. 4C is the clue to the key.
Other things that spring to mind:
Why Charlie, X-Ray, Yankee as breaks? Could have no meaning at all. But 'C' tends to invite an 'AB' and 'XY' invites the Z. The C is the last of the obvious pattern and the X and Y the first two of another. May be that is relevant. Or not.
3, 5, 6 are the only other numbers. Is the message the numbers, or the ones and their frequency, perhaps?