3 posts is for unmoderated linking (mentioned in the notice for new users), 1 post is for "sorry, your requested thread is not available" (no mention of it, and i haven't noticed any pattern in the affected threads yet or anything that'd make those threads undesirable for unregistered users to view so i presume it's a xenforo bug)
haven't noticed any pattern in the affected threads yet or anything that'd make those threads undesirable for unregistered users to view so i presume it's a xenforo bug
Otherwise, not sure why new users with <3 posts can't access and/or view certain threads, but they can for others. As you mentioned, there doesn't appear to be any sensitive material in the affected threads that would indicate anything "inaccessible" for either unregistered/new users to view. So its somewhat murky territory to me at-least.
Might warrant the need for further investigation if needed in the near term.