When I created my first character I unlocked subclass at level 15.

My third character which was the same class as my first char I selected my subclass at level 5.
Can someone explain this please?

Could I select my subclass for my third char at really low level because I has already unlocked it on my main?

Once you unlock your subclass for your first character it's unlocked on all characters, including those you haven't made yet.

Oh. That's pretty cool, I thought it was something like this.