Modal combination refers to addition of modal values .Total soln. is a sum of the modal solutions ( maximas in the response spectrum concept).

Directional combination refers to addition of values from same directional excitations like say you have two excitations in X direction.

Spatial combination refers to addition of values from different directions like say X and Z.

I would recommend a SRSS combination method for Spatial as they show "statistical independence"( I don't think the concept of "directional" is applicable in your system as your system is excited in X, Y and Z directions with no multiple excitation in the same direction)and a DSRSS method for modal combination with modal preceeding spatial.
Use ABS combination for SUS and OCC cases.

Regards