When a plan of reclamation has been adopted by a district located entirely outside the boundaries of the Sacramento and San Joaquin Drainage District the board of supervisors shall appoint three assessment commissioners, who have no interest in any real estate within the district, each of whom, before entering upon his duties, shall make and subscribe an oath that he is not in any manner interested in any real estate within the district, directly or indirectly, and that he will perform the duties of a commissioner to the best of his ability.