how to make 3d texture mipmaps

Hello guys, I am using 3d texture to make terrain, so I can use blend on fixed pipeline, but I have a problem that mipmaps looks bad (cause 3d texture), you can clearly see the transition line and also on some materials, on example sand that has grass and rock near it, doesn’t even show in distance when mipmaped. Is there anything I can do? Preferable at as much lower OpenGl version as possible.