You'll have to fill in the last spot ox x2 with something if you want to combine x1, x2, and x3 into a single matrix. The value you choose would depend on what the matrix will be used for. One option is to fill with not-a-number (NaN)
x1=[9 6 9]';
x2=[0 2]';
x3=[3 1 2]';
X = [x1 [x2;nan] x3]
