You don't. Your i and j and k are not referring to imaginary variables, they are referring to mutually perpendicular direction vectors. You can emulate the form A*i + B*j by using the complex value A + B*i or A + B*j which are identical (i and j mean exactly the same thing in MATLAB.) MATLAB does not, however, provide syntax for extending this notation into another direction.
The closest you can get without loading in a package from the File Exchange would be coding something like
U = [-8 -14 +25]
and then taking care with the operations done on the vector.
