Even if you grew up on a station and had been mustering on the ground for years you couldn't be over qualified for mustering.
Grade Threes require supervision and mentoring. Most schools know that a Grade Three will disappear to one of the mobs who won't hire Grade Threes as soon as they get the Grade Two so it's not worth investing all that time and expertise just so the big internationals can benefit.
Sounds like you have been given some unrealistic expectations.