Try this:
a = randn(1,31);
b = randn(1,31)*2 + 2;
c = randn(1,31)/2 + 3;
figure(1)
boxplot([a(:) b(:) c(:)])
set(gca, 'XTickLabel', {'a','b','c'}) % Optional
It forces them to become column vectors, horizontally concatenates them, and plots them.