Don't know if it's still the same but when I was in the army, Sandhurst graduates would join their units with the rank of 2nd Lt. So this lady has gained another two pips on her shoulder in just 3 years. I suspect that her prowess in the sporting field may have played some part in this which, back in the day, was quite the norm.
Not wishing to detract from the brilliant achievement of this young lady but if the Army promotion system is the same as the RAF then her rank of Captain, equal to Flight Lieutenant, is an automatic promotion and attained after 2.5 years.