A mutation conveying a 3% advantage (which is pretty darned large, as mutations go) has a 6% chance of spreading, at least on that occasion.2 Mutations can happen more than once, but in a population of a million with a copying fidelity of 10-8 errors per base per generation, you may have to wait a hundred generations for another chance, and then it still has only a 6% chance of fixating.
