Weird, TileMap provides them but not its 3D counterpart, apparently.
Anyway, you can get the conversion between world and grid-space with these formulas (a bit simplified), assuming your world node is centered at (0,0,0) with a scale of (1,1,1):
var v = pos / tile_size
return Vector3(floor(v.x), floor(v.y), floor(v.z))
var v = pos * tile_size
# This second calculation is to center the position on the tile
return v + tile_size/2.0
If your game is fully grid-based, you can create your own
grid_position variable in your objects using a script for your game logic, and convert to world coordinates when you want to change the position of the node. So to advance 1 cell forward, you would do
grid_position.z += 1, then
spatial.set_translation(grid_to_world(grid_position)) to update the node.