The acceleration is a 3D vector, which has a length (or magnitude) and points in some direction. So, if you want to know about changes in acceleration you have to consider changes in both the length and the direction.
The length L = sqrt(xx + yy + z*z)
The direction cosines in x, y, and z (which give the information about where the vector is pointing) are given by
dcx = x/L
dcy = y/L
dcz = z/L
Google will tell you lots more.