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There isn't any law against treating people differently based on what shift they work. The laws on discrimination are actually very limited. Paying people differently based on race, religion, gender, age, disability or FMLA use is illegal. Paying people differently based on shift is not.
Now, if there is some contract with the employees that requires 3rd shift to receive comparable pay, then that could create a contractual issue...but the rights would be based on that contract and not on any external laws that make it automatically illegal.