Student or Learner
"What time is it now?" "It's exactly three."
Do you use "just" in place of exactly?
It's common to say "just about", as in "It's just about three now", which means that it will soon be three.
Saying "it's just three" would mean "it's only three", but I think "it's only three" is more likely to be spoken than "it's just three".
It's only three? We have a few more hours left.
On further reflection, I think I would probably say "It's just 3 o'clock" rather than "It's exactly 3 o'clock". Both are correct of course.