Godot is a completely free, open-source, cross-platform, and lightweight 2D + 3D game engine. It started as a 2D game engine, but later expanded its capabilities for 3D game development. Compared with many other game engines, it is small in size, rich in features, simpler and easier to use, and more friendly to the novice game makers. The games made with Godot can be simultaneously released to Windows, Mac, Linux, Android, iOS, UWP, and HTML5 platforms.

From a professional perspective, Mathematics for 3D Game Programming and Computer Graphics explains the mathematical concepts used in 3D Computer Graphics and game engine that a game developer must master. Compared to the second edition, the third edition expanded on the content, introducing the knowledge for projection, shadow, physics, cloth simulation, and numerical methods. And all chapters have been revised and the shader code has been modified according to the new specifications of the shading language.

To learn 3D game development with limited pockets, I guess you've tried many classic, free, open-source game development engines like TV3D (Truevision3D), OGRE and Irrlicht Engine. By contrast, the pros of TV3D are easy to learn and get started with; its cons are that it was built based on the ActiveX control and requires the RunTime DLL to be registered before running; OGRE is better than Irrlicht in that it has a capable and vigorous engine framework, and supports for extending new functionality in the form of plug-ins (this is better than Irrlicht); but its downside is that it's not easy to get started with.

Torque 3D (full name: Torque Game Engine, TGE for short) is free, open-source and cross-platform 3D game engine developed by GarageGames (originally developed by Dynamix for the first-person shooter game Tribes 2 released in 2001) under the MIT license. It features powerful and complex rendering system, high-speed network transmission, authentic real-time editing, script editing system, as well as maximum flexibility and performance across a wide-range of hardware, and a full suite of tools for producing high-quality Windows and browser-based games development.

Dungeon Siege is a very classic series of action RPG games developed by Gas Powered Games (now called Wargaming Seattle) and published by Microsoft Game Studios in 2001. It comes with 2 sequels: Dungeon Siege II (in 2005), Dungeon Siege III (developed by Obsidian Entertainment and published by Square Enix in 2011, supervised by the founder Chris Taylor from Gas Powered Games).