Is 2 KClO3 → 2 KCl + 3 O2 balanced?

yes it is balanced.

Explanation:

when you are balancing an equation you look at the elements or compounds on the reactants and product side, which must have the same number of elements on both sides. like the question above;

2 potassium on the reactant and product side.

2 chlorine on the reactant and product side.

6 oxygen on the reactant and product side.