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.

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.