All three of these cases included exceptions to the exclusionary rule.
In United States v. Leon, the good-faith exception was added to the rule, which states that evidence obtained with a defective search warrant can still be used at trial if the officers collecting said evidence were acting in 'good faith'.
Massachusetts v. Sheppard further enforced this rule.
Nix v. Williams introduced the inevitable discovery exception, which would allow otherwise inadmissible evidence that proves the defendant's guilt to be used as evidence in a trial.