As far as I know, you can get the screen size and density using Gdx.graphics.getDensity(), so you can load the right texture for E.g 1x, 1.5x etc..
but what about the texture that comes with the 3D model, for E.g. the texture is only intended for a maximum 1280x800px, while my android has dpi 3x.
I don't want to scale it too much because it can cause the image become too blur/fade/not sharp, anyone who knows the solution please?
EDIT: let me explain in detail
so in my opinion the solution is to make some textures atlas for 240dpi, 320dpi, 480dpi etc. the problem is I don't know how to replace the texture atlas which from the beginning already integrated with the Model? so when scaling up, texture atlas is automatically replaced with a higher one. thanks
Usually in 3D graphics the camera or model are mobile. There isn't a fixed best resolution of a texture because the camera may be very near, very far away or viewing the textured surface at a glancing angle.
The solution offered by graphics APIs are settings for texture filtering. Under magnification (where a texel takes up more than a screen pixel) you can do linear filtering for soft edges or point filtering for hard edges. Minification is more complex, you can have linear or point filtering, but you can also have mipmaps, which are a precalculated chain of successively half-sized versions of your image, typically all the way down to 1x1. You can set texture filtering to pick the nearest mipmap, blend between mipmaps, or use anisotropic filtering for better sharpness at glancing angles. Generally linear filtering for magnification, and full mipmap chains with anisotropic filtering for minification produces very good quality and good enough performance to be a good default choice.
So, you won't be giving the GPU a single texture for your model, you'll be giving it a chain of textures, and letting the GPU worry about how to sample that chain to give the correct amount of blur/sharpness. For performance and compatibility with mipmaps, it is usually a good idea to use power-of-two textures (e.g. 1024x1024 rather than 1280x800).
So, just make a 1024x1024 or 2048x2048 texture with mipmaps and appropriate filtering settings then use it on every device regardless of its resolution then quality-wise you're sorted.
If you're particularly worried about memory use or load times then there's an argument to reduce the texture size on lower resolution devices (basically, have a second asset with halved resolution for low-res devices, or just skip the highest resolution mip when loading for low-res devices), but I think that might be a premature optimization at this stage.