Very often, people who are just starting to use MATLAB, think that if you don't know the value of something, then it must be symbolic. That is not always true. I'll give some examples later. But to do what you are asking right now, just do this:
X = sym('X',[1,3])
X =
[X1, X2, X3]
As I said, you can have an unknown that will then be estimated. For example, if we stay in the purely numerical domain, we can solve for an unknown vector. This eample is pretty simple, but...
fun = @(X) sum((X - [1 2 3]).^2);
Xinit = [4 4 4];
Xfinal = fminsearch(fun,Xinit)
Xfinal =
0.99999144502901 1.99997109582809 2.99995541073201
Of course, the minimum arises at the vector [1 2 3].
