Here is a method I've just invented for describing piece movement within the 3D matrix (board) that you may utalise or find convenient for describing piece movement. Feel free to improve upon this method.
Vector
Using the notation: (x, y, z), where x describes the horizontal axis (equivalent to the alphabetic "files" in standard chess), y describes the depth axis (equivalent to the numeric "ranks" in standard chess), and z describes the vertical axis (described using greek letters in conventional 3D notation). We can describe a piece's movement using classical vector notation (albeit the movement is described within a 3-dimensional matrix).
Many pieces can move in different ways, so to keep things as thoroughly identifiable as possible, the variables that a piece can move are described explicitly in the direction in which they can move. For example, if a piece's movement were described with the following:
(1, 0, 0)
This would mean that the piece would be able to move 1 space horizontally (left or right) upon movement.
To describe a piece with move versatility and to include other variables of movement direction we would use the & symbol to string movement capabilities together.
Example:
(1, 0, 0) & (0, 1, 0) & (0, 0, 1)
This describes a piece that can move one space horizontally (left or right), one space depthwardly (forward or backward) and one space vertically (up or down).
We use the r symbol to indicate a rider, this may appear within brackets or outside a bracket depending on necessity.
(r, 0, 0) & (0, r, 0) & (0, 0, r),
r(1, 0, 0) & r(0, 1, 0) & r(0, 0, 1),
Both of the descriptions above describe the movement of a Rook in 3D-Grandchess.
Capture capabilities are generally described as the same for movement. However, if the piece can capture in a way that it cannot usually move, we describe this "capture bracket" with the symbol c. If the piece can move in a particular but cannot capture as such, then we describe this movement with the symbol m. Here is a description of how a pawn in 3D-Grandchess operates (excluding en passent, promotion and 2 square first move as I haven't thought that far ahead yet):
mF(0, 1, 0) & cF(1, 1, 0) & cF(1, 0, 1) & cF(0, 1, 1)
In the above example we see use of the symbol F. This refers to the fact that the movement/capture can only be completed "forward" in the "y" direction.
To describe movement that is only specific to a particular direction we can use the following:
F = forward
B = backward
L = left
R = right
U = up
D = down
FL = forward-left
FR = forward-right
FU = forward-up
FD = forward-down
BL = back-left
BR = back-right
BU = back-up
BD = back-down
LU = left-up
LD = left-down
RU = right-up
RD = right-down
FLU = forward-left-up
FLD = forward-left-down
FRU = forward-right-up
FRD = forward-right-down
BLU = back-left-up
BLD = back-left-down
BRU = back-right-up
BRD = back-right-down
Using this system we can describe the movement of many pieces in 3D chess boards with clarity. The pieces in 3D grandchess can thus be described as follows (except the pawn in detail and castling stuff because: hard):
King (without castling)
(1, 0, 0) & (0, 1, 0) & (0, 0, 1) & (1, 1, 0) & (1, 0, 1) & (0, 1, 1)
Queen
r(1, 0, 0) & r(0, 1, 0) & r(0, 0, 1) & r(1, 1, 0) & r(1, 0, 1) & r(0, 1, 1)
Rook
r(1, 0, 0) & r(0, 1, 0) & r(0, 0, 1)
Bishop
r(1, 1, 0) & r(1, 0, 1) & r(0, 1, 1)
Knight
(1, 2, 0) & (2, 1, 0) & (1, 0, 2) & (2, 0, 1) & (0, 1, 2) & (0, 2, 1)
Pawn (without en passent and first move allowing for 2 square movement)
mF(0, 1, 0) & cF(1, 1, 0) & cF(0, 1, 1)
In a play like Chess a Piece if defined by how it moves; i.e. what part of the board it can reach, which squares and how many of them. Ultimately, what defines a piece is its mobility
Just to be precise these are the characteristics distinguishing the figures, or types of pieces. Then we may have a number of copies of any figure and those are the Pieces.
Greater mobility gives the pieces more opportunities; therefore it makes them stronger for the purpose of the game. We all know that the Queen is the strongest figure in chess because it's the one with greatest mobility; while the Pawn is the least strong because its mobility is the lowest one.
Adding one more dimension, we multiply the spaces available for pieces and we also open up new lines and directions that they can use for their mobility. Because of the increased mobility they are hence defined differently: they are in fact Substantially Different Pieces, significantly More Powerful than those we know in chess.
In addition to knowing how to orient ourselves and operate in the three dimensions, not at all an easy task for those used to play chess, we must also learn to handle different and much more powerful types of pieces.
This will require specific training for the capabilities of a human mind willing to engage in these types of games. But this is the same kind of capabilities needed to govern the complexity of the world, today and in the near future.