The third time wasn’t the charm for these two suspected diamond thieves.

The bandits were busted after they swiped nearly $20,000 worth of diamond charms and rings from two UPS trucks and a Rockefeller Center office tower, authorities said yesterday.

Following thefts on May 10 and 19, the suspects, identified by police as Pedro Montanez, 36, and Juan Rivera, 45, allegedly teamed up Friday to steal four packages containing $6,244 worth of diamonds from a UPS truck on West 48th Street near Sixth Avenue.

But they were foiled in their getaway by a cop, who had seen the pair casing the truck for 45 minutes.

After Rivera stole the packages, the officer quickly moved in.

The two were charged Friday with grand larceny.