Why do you specifically want to use a 3D Texture? It seems like that’s not ideal for terrains which aren’t likely to change shape dynamically (eg, dynamically carving out a solid like wood is more suited for 3d textures).
Typically what you do is create a shader with several textures then blend/mask between them at different elevations and slopes. I do this in conjunction with landscape painting layers and it comes out real nice, I even have a nice, patchy transition between grass and rock. If that might be useful for you, you can get a copy of the terrain shader here: A different kind of cel shader - Work in Progress - Unreal Engine Forums