There might be a sticky situation buried in the way you have to treat day, night and graveyard shifts. Night shifts might pay a little higher than day shifts, and graveyard (overnight) shifts might pay even higher than the night shifts. This extra money is a matter of agreement between the employer and the employee, and that’s fine.
The gray area happens around the threshold boundaries for those type of shifts. If you define the day shift as: “anything that starts earlier than noon,” and your second shift with the higher pay-rate is: “anything that starts at noon or later,” it can create a source of contention for those who start at 11:30 a.m.
It’s better to cap your day shift start time at 11 a.m. or earlier (when you can), to create a broader distinction with that extra pay incentive. Also, be careful about granting special break situations to the second and third shifts that you do not allow for first shift (e.g., skipping the entire unpaid and paid breaks and shortening the overall span). While it may alleviate those extra-low occupancy levels in the middle of the night, when the other agents find out, they might feel slighted—especially since the graveyard shift may be getting the premium parking spots, too.