3 point perspective is something that you will likely not use as much as 1 or 2 point perspective but it can still be useful to know especially if you are creating a image of someone looking up at or down at a building. If you have looked at my earlier post on 2 point perspective you will find that adding a third point is fairly simple as may of the same ideas apply when using 3 point perspective.
To demonstrate drawing an object in 3 point perspective I will as in previous posts I draw a simple cube. The cube will be drawn as if we are looking down on it just as was done in my earlier posts in perspective.
As in the previous posts on perspective I have started with an horizon line (in blue) and some vanishing points (in green) 2 of which are on the horizon line the same as if drawing using 2 point perspective and a new third vanishing point at the bottom of the image. The initial perspective lines have been added (in grey) to act as guides for adding the top of the cube.
Using the vanishing point at the bottom of the image you draw some perspective grid lines from the vanishing point to three points on the top of the cube, these points are the front and side corners. what this perspective grid does is give you the sides of the cube as you will see later.
Now we need to determine the length of the cube using the perspective grid line from the bottom vanishing point to the front corner of the top of the cube as a guide. What you need to do now is using this line add 2 more lines to the other 2 vanishing points to indicate the bottom of the cube.
After the 2 lines for the bottom of the cube have been added you have solid cube, if that is all you wanted you could stop here but if you wanted a cube that you could see through (wire-frame) then there are only a few simple steps that are needed to accomplish this.
To add the lines that would often not be visible first add a perspective grid line to each of the vanishing points on the horizon line from the opposite bottom corner of the cube. This will create an intersection point and from there draw a line to the top corner at the back of the cube. if you wanted to you could extend this perspective grid line all the way to the bottom vanishing point as that is the vanishing point that the line originates from, but I have not shown this in the above image.
This was the last of the different types of perspective commonly used this is still just the beginning of using perspective to create drawings if you want to learn more there are two book I would recommend the first is Perspective made easy by Ernest R. Norling and the second is Successful Drawing by Andrew Loomis. Once you have got a good understanding you may also want to have a look at How to Draw by Scott Robertson.