Very cool Alberto! This is a difficult subject and feels sort of like math (which no one seems to like much!). Great that you are putting in the time to master it.
There is one area that you should really work on early so you don't practice the wrong thing. Distortion in linear perspective is a real pain, but is easy to overcome with a few principles. The more points you add, the more this stuff matters (ie, two point won't distort as much as three point).
In order to make this easier to understand, I did a quick draw over. I pointed out in yellow the boxes that were starting to get away from you. The reason these shapes distorted is the points were too close together and the shapes actually went outside of the usable area. This can't happen in reality. The easy fix is keep your image way inside the vanishing points and you will be safe. An added layer of safety in three point perspective is keep your horizon line out of the picture.
I made two boxes in the top right illustration. both boxes use the same vanishing points. Box A stays inside the points, box B goes outside the points. You can see how it starts to get kinda wacky out there in no mans land.
The illustration at the bottom is how I would recommend practicing 3 point perspective at the beginning. Keep the points as wide as you can. Even off the page. The result is I actually can't draw outside the usable area and the shapes won't distort.
Hope that helps some! Keep up the great (and difficult!) work. : )
Cheers,
Lee
