From experience the longest part is to dig the ground to pour the concrete to ancker the thing. And don't skip this as kids can be rough on it when growing up and it cannot stand safely without proper ankers.
Did a similar 3 level one - took me 18 hours with 8 hours of help from a guy that needed the labor and my BiL and friend another 3 hours. Also had the advantage that the wife and kids were out of town.
Good luck!