There are in fact various 3D rules that can be made to behave exactly like Conway's Life -- starting with some discoveries by Carter Bays involving a two-cell-thick "sandwich" of Life cells that emulated standard Conway's Life. Without the sandwich trick it can do other interesting things:

http://www.complex-systems.com/pdf/04-6-2.pdf

The two problems with 3D life have always been

1) 3D is computationally very expensive compared to 2D, so you tend to get stuck working with much smaller grids; and

2) you need a really good visualization system, otherwise interesting patterns and uninteresting patterns both look like ugly random blobs of cells and you can't tell what's going on.

That said -- not promising anything, and especially not on any definite timeline, but the next version of Golly might have a Lua-based system for experimenting with 3D CAs.