Reading and understanding someone elses source codes is more difficult then make a simple engine yourself.

You could try Ogre3D engine. It is opensource, C++ based, I have worked with it and it is just fantastic. Though not the easiest to learn and it hasn't got an editor like Unity3D does. I hope this helps you!

You could try Ogre3D engine. It is opensource, C++ based, I have worked with it and it is just fantastic. Though not the easiest to learn and it hasn't got an editor like Unity3D does. I hope this helps you!

Keep in mind that Ogre is a graphics engine, which is only one small component of a game engine.

I'm going to make a switch from Maratis to the recommended Godot Engine from this post. Now I put as the best free and open sourced alternative to Unity3D. Just as I watched Game Maker and Unity grow, I am excited to see this grow. Hopefully they don't go expensive like the Game Maker and Unity did.