Hiya! I hope it's okay if I start from the box section, since I've already looked at (& commented on) parts 1 and 2.

So, first things first- please don't use lined paper. It's fine for some exercises, but for others, like the rough perspective exercise, it's super cheating. Regarding the exercises themselves... The plotted perspective exercise looks good! The rough perspective exercise looks quite good, too, but in one of the pages (cough...), you'll have some trouble making the parallel/perpendicular lines be parallel/perpendicular. Remember that it's important for them to be, because those VPs are at infinity. Also, be careful to not correct incorrect lines. If a line is wrong, it's wrong. Spend some more time ghosting the next one, instead. Nice job on the rotated boxes exercise! As for your question, at this stage, we're constructing boxes by looking at their neighboring forms, nothing else. We don't start to think about them on an individual level until the box challenge, so, for now, don't worry! Finally, the organic perspective exercise looks good, too, but here, too, be careful not to redo lines. Also, line-weight should be applied locally, rather than to the entirety of the line. (So, clarify the overlap, then taper it off!)

Anyway. Despite all of this, this is a solid submission. Nicely done, and good luck on the box challenge!