First things first, mathematical concepts: undertanding vectors and matrices. You will learn how to operate with both as well as understand some relevant concepts like the difference between dot and cross products. Of course, further separated reads can be found on vectors and matrices.
Assuming that you've got that clear, you can then move onto the next concept: coordinate systems. What's the difference between local and world coordinates? What's tangent space? You'll get a first glimpse to these concepts, dont' worry.
Now enters transformation matrices. So, what's a transformation matrix? You will learn that in order to get the desired and or correct 3D projection, the order how you operate with matrices is the key.
Ok, let's take a larger step here, ... this topic is not particularly referred to 3D only, but it comes very handy when you want to build a game engine and start dealing with parent and children objects moving along together and thus, with matrices concatenation: scene-graph.
Implementing a scenegraph varies on a per-project basis, but if you understand the basic concepts you will do well, even if you don't (want to) call it a scene-graph in your source code or want to avoid its whole implementation but still preserve its functionality.
Well, as you can see, this is just an ad-hoc primer, built by just browsing the web with the help of search engines, so if you don't have money to buy a good book on the subject you will find that Internet is a great place to find the information you need if you're patient while searching.Have a nice read ...