Employers who operate a three shift system would normaly have what's known as a shift allowance to compensate (in part) for unsociable working hours. Obviously the later the shift the bigger the allowance, but it's not a given. A very long time ago when I worked a day shift for a company, it was succesfully argued by those on the day shift that just because people worked nights this shouldn't entltle anyone to extra wages. This was on the basis that everyone on nights was a volunteer for the shift, and regardless of when they worked they still only worked 8 hours out of 24. naturally it went up like a lead baloon with the night shift workers but the management withdrew the shift allowance (clearly in their interest financially) anyway.