Just to clarify, the transfer function for a MIMO system will actually be a matrix of transfer functions. Lets say you have MIMO system with two inputs u1, u2 and two outputs y1, y2 then you will have total four transfer functions as follows:
Y1(s)/U1(s), Y1(s)/U2(s), Y2(S)/U1(s) and Y2(s)/U2(s).
The code in your case would be as follows:
% firstoff you would have to define 'a' for now I will just assume a value for it as I am unaware what 'a' is.
a = 1;
A=[-1 0 0 ; 1 -a 0 ; 0 1 0];
B=[1 0 ; 0 a ; 0 0];
C=[0 1 0 ; 0 0 1];
D=zeros(2,2);
[num1 den1] = ss2tf(A,B,C,D,1); % iu = 1
[num2 den2] = ss2tf(A,B,C,D,2); % iu = 2
sys = ss(A,B,C,D);
tf(sys)
