I have copy-pasted your code below, exactly as entered in your question. It runs and produces the plot with no error. I suggest that you double-check your code carefully. If you still get an error, then copy your code from MATLAB directly into a comment here, along with the exact text of any error message, so that we can take a look.
x = [ 1, 2, 3, 4, 5]
x1 = [ 1.1, 2.2, 3.4, 4, 4.5]
x2 = [0.9, 1.9, 3, 3.9, 4.3]
y= [1, 2, 3, 4, 5];
y1=[2, 3, 4, 5, 6];
y2 = [1, 3, 6, 7, 8]
hold on
plot(x,y)
plot(x1,y1)
plot(x2,y2)
hold off
