So if you just scale your coordinates to fall into the range 0 <= x_i < 64 and take int, it should work…
I’m not sure whether scatter_mean supports multidimensional indices, so you might then multiply x_0 * (64 * 64) + x_1 * 64 + x_2 and to get a 64**3 tensor from scatter_mean that you can then view as a 64, 64, 64 tensor.
Best regards
Thomas