If you are interested in compiling your own Android export templates, see the Compiling for Android documentation.
Note that Godot 3.0 requires OpenGL ES 3.0 to be supported on target devices. Unfortunately, the quality of OpenGL implementations on mobile GPUs varies wildly, making it difficult for engine developers to make sure the engine works well on most devices. Additionally, as of writing, over 35% of Android devices currently in use only support OpenGL ES 2.0. (Note that this statistic is to be taken with a grain of salt, as not all Android users are interested in mobile gaming.)
Godot 3.1 will reimplement an OpenGL ES 2.0 renderer aimed at mobile devices and low-end desktops/laptops; you can read a progress report here.