If you're in your third year, surely you only have two and a bit years' experience so far? You can't count your PGCE, I'm afraid. Three years is not nearly enough to be a deputy head. The job means you're completely ready to be the head - with all the responsibility that entails - on day 1 if the head falls under a bus. Your Master's degree is neither here nor there in terms of readiness to he a deputy. I suggest getting at least three years' more experience, preferably in a different school. Even then, six years is very little compared with the seeming eternity you might remain in teaching.