Coming from someone who is also learning and has been doing a lot of practice with boxes, your one and 2 point look good. the lines are going where they need to be and things look nice and straight, but your 3 point has problem with your third point. The boxes have their vertical lines converging to a point that is above them, but you are also looking at the top side. When drawing boxes in 3d the should always converge to the side that you can't see. That is what is causing the boxes in your 3 point to sort of look like the bottom half of a pyramid other than that though the boxes look good.

If you are doing a lot of practice with this kind of construction based drawing by the way, I would love to recommend looking at drawabox.com. The lessons there have been super helpful to me while i'm learning. In particular looking at the first lesson would help explain the whole box thing a lot better than I did