It depends on your goals. I'm actually developing a 3D game engine of my own. Let me tell you this: It is extremenly difficult to develop a game engine.
There are so many things that you need to learn. For example: Linear Algebra, OpenGL, Shading Language (GLSL), Quaternions mathematics, Design Patterns, Data Structures/algorithms, Object-Oriented design, C++, Python. You also need to get acquainted with tools such as Blender.
My goal was to Learn and I'm glad that I decided to develop a game engine. It has been an amazing experience. I have learned a lot. Not only on computer graphics and programming but also about myself.
If you decide to develop one, realize that this will feel like a Marathon. You will want to quit, you will have ups and downs but in the end you will be glad you did.
I started developing the game engine in early 2013. It has been 2 years of development and there is still a lot to do.
If your goal is to release a game as soon as possible, then do not develop a game engine. If your goal is to learn and are willing to put in the time, then do so.