Nice work on the studies
Umm as for the 3 point perspective, I guess it’s more of a concept thing to wrap one’s head around as it can be a bit ambiguous or confusing at times. From what I have learned up to now I can say its a bunch of technical rules that apply to line and object placement in art that help sell the way humans perceive 3D space. Those rules will vary delending on various factors that change from scenario to scenario
For example, when looking straight ahead towards the horizon, if an object is placed in front of you such as what you drew last, that is tilted only to its side (rotated on the Y axis only) you would apply 2 point perspective. When applying the vanishing points, the closer you place them the more the geometry gets compressed at the middle like a wide angle camera lens and the farther appart you place the points, the more you simulate a telefoto lens with no deformation
If you tilt the object on the X axis, or if you start looking at an angle upwards or downwards, you would have to use 3 point perspective to indicate that in your drawing
So for your example, what you can do is make some front and side view schematics and then imagine placing yourself closer to the object and looking up, or being on top of the wall and looking down, then you will have to use the 3 point perspective rules
Dang that was a lot of text, hope it helps
Cheers!