3 point perspective is infinitely more useful for creating more dynamic drawing compositions because, in real life, there is always more than two vanishing points. This exercise is a guide on creating a flexible third vanishing point that can be moved around at will.
First, the artist must determine the size of the canvas. Ideally, draw the 3 point perspective grid on a very large canvas as the end goal is to move the perspective lines into a smaller canvas. This is to reduce any distortion due to the vanishing points being too close to another.
Speaking of vanishing points, the very basics of a vanishing point in a drawing can be simplified as a series of guide lines coming out of a central area. The best shape to visualize this would be a stylized star burst. Surprisingly, it's not that draw at all.
The key is to treat this shape as a series of single lines rotating around like a wheel. When there is enough lines intersecting, it will create the star burst shape automatically. Using very simple tools, here is how it's done in Photoshop.
Make sure that the canvas is a perfect square. On the canvas, start with just with a single horizontal line and a single vertical line. The end result is a division of the canvas into quads. If these are done on separate layers, merge the lines together.
Duplicate the layer and then start rotating it. Holding on to the 'shift' key will rotate it 15 degrees in any direction. When done, merge the layers together. Repeat the step until there is an adequate amount of lines starting to form a circular pattern.
Up to a certain point, all the 15 degree rotations will be taken. When that happens, duplicate the layer again, but this time, do a manual rotation of about 5 degrees or so. In Photoshop, there is an angle window that allows the input of individual values.
This only needs to be done for the first few lines. After that, duplicate and then flip it in both horizontal and vertical positions to quickly fill in the rest of the lines. From there, merge all the layers together and then right click it to select 'Convert to Smart Object.'
Changing the star burst shape into a smart object will allow scaling, up to its maximum size, without the pixelated effect. This is exactly why it needs to be done on a very large canvas. For the rest of the guide, duplicate the star burst into 3 separate layers.
Each of these star bursts will represent 1 vanishing point. Having three of these will mean that are 3 vanishing points waiting to be arranged to form a 3 point perspective grid. Before that, color code the layers so it becomes easier to see.
In Photoshop, simply color the lines using the 'color overlay' layer style. For visibility purposes, green, blue, and red are used for each of the vanishing points. In terms of arrangement, 2 out of the 3 vanishing points must meet with another to form a horizon line.
In this example, blue and red are used to form the linking horizon line. This green then becomes the free floating point for the 3rd perspective. For now, having it set to anywhere on the horizon line creates a very accurate 3 point perspective.
Since we are dealing with 3 point perspective, the green can be shifted anywhere below or above the horizon line. The most common and natural placement will be above the horizon line. The reason for this is how we see the world around us.
For the majority of the time, our field of view is from ground level. When looking at any scenery, the closer the object, the easier is it to see that the perspective of the object narrows towards the top if we are looking at it directly.
In a drawing, the green vanishing point will be placed outside of the canvas much further away than the red and blue vanishing points. The important hint to note is that the drawing is contained within a diamond formed by the red and blue vanishing points. This prevents distortion.
Therefore, when importing the perspective into a smaller canvas, make sure that distance between the red and blue vanishing points are at least triple the width. For example, if the canvas is 1000 pixels wide, then the distance between the blue and the red points should be around 3000 pixels.
While it is easier to create a drawing looking up based on ground level, it's harder to draw a perspective looking down. To be precise, there is a risk of too much distortion when looking from ground level downwards.
The reason is due to how perspective works in the real word: there is actually a 4th vanishing point. Unlike looking upwards, there is no ground level to cover up any distortion caused by this extra vanishing point. There are two methods to resolve this.
The more accurate method would be to introduce the 4th vanishing point so items like skyscrapers can naturally 'bend' towards a point up in the sky while looking down. Just think of it like a curved lens. The other method in resolving this distortion is to shift the entire horizon line out of the drawing.
An interior drawing like a top down view of a kitchen, for example, no longer shows the horizon line. Instead, it is shifted upwards in an attempt to get rid of the extra vanishing point. In doing so, we are now left with a simple 3 point perspective--like how it is originally intended.
Regardless of how the lines are placed, it all comes back to creating a flexible system that allows perspective lines to cover as much of the composition as possible. This is the reason why the 3 point perspective has to be done on a much larger canvas.
With that said, since this is done manually, it means that there is no reliance on any single digital art tool, which means the artist is not locked to a specific drawing software. Even then, the perspective drawing can be exported as a picture file to be used in any other drawing software. That's flexibility!