I've read a claim in a few places that Rockstar confirmed the third meteorite is in the game, but I can't find anything from Rockstar or any reputable gaming outlet to back it up. Additionally, there were a couple posts on Reddit that claim a conversation can be overheard in Valentine about someone's daughter finding the third meteorite, but I've never heard it myself and ... well, it's Reddit.
I'm also not finding anything credible that links it to GTA Online, so I'm pretty sure you're good there.