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 https://imgur.com/cPh1tmr 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.