First, Harry is at least aware that Hogsmeade exists and that the faculty sometimes visit - it's brought up in Philosophers Stone, just not by name (though the Hogs Head pub is).
"Won it," said Hagrid. "Las' night. I was down in the village havin' a
few drinks an' got into a game o' cards with a stranger. Think he was
quite glad ter get rid of it, ter be honest."
"It's not that unusual, yeh get a lot o' funny folk in the Hog's Head
-- that's the pub down in the village."
It is unclear whether Harry knew that students could visit, but he clearly knows Hogsmeade by name; when he reads the letter, his reaction is more focussed around getting the permission slip signed (either he didn't know they could visit OR he assumed they didn't need permission from guardians).
Harry pulled out the Hogsmeade permission form and looked at it, no
longer grinning. It would be wonderful to visit Hogsmeade on weekends;
he knew it was an entirely wizarding village, and he had never set
foot there. But how on earth was he going to persuade Uncle Vernon or
Aunt Petunia to sign the form?
Secondly - a broad answer would be that Ron does know this stuff (Hogsmeade anyway, generally only the adults know about Sirius), but doesn't mention it to Harry because he assumes the others already know (sometimes it's hard for him to remember how much of their lives are different from his, and what he thinks of as "basic knowledge" doesn't apply to them).
I think the best example is in book seven, when discussing Wizarding fairy tales.
"And as for this book." Said Hermione, "The Tales of Beedle the Bard
... I've never even heard of them!"
"You've never heard of The Tales
of Beedle the Bard?" said Ron incredulously. "You're kidding, right?"
"No, I'm not," said Hermione in surprise. "Do you know them then?"
"Well, of course I do!" Harry looked up, diverted. The circumstance of
Ron having read a book that Hermione had not was unprecedented. Ron,
however, looked bemused by their surprise.
"Oh come on! All the old
kids' stories are supposed to be Beedle's aren't they? 'The Fountain
of Fair Fortune' ... 'The Wizard and the Hopping Pot'... 'Babbitty
Rabbitty and her Cackling Stump'..."
"Excuse me?" said Hermione
giggling. "What was the last one?"
"Come off it!" said Ron, looking in
disbelief from Harry to Hermione. "You must've heard of Babbitty
"Ron, you know full well Harry and I were brought up by
Muggles!" said Hermione. "We didn't hear stories like that when we
were little, we heard 'Snow White and the Seven Dwarves' and
"What's that, an illness?" asked Ron.
So sometimes he doesn't bring up stuff unless Harry asks or is visibly stumped. Hermione has a similar, though less pronounced problem - she's extremely diligent in her studies, and assumes everyone is else is just as neurotic and prepared as her - as evidenced more than once with the recurring joke about Hogwarts: A History.
"Honestly, am I the only person who's ever bothered to read Hogwarts,
A History?" said Hermione crossly to Harry and Ron.
So, at least for Hogsmeade, it's likely that one, both, or all three knew. However, since it wasn't relevant to the present, and they assumed everyone knew about it, they didn't bring it up.