DEAN MAZZARELLA’S day starts at 3 a.m. when he hops out of bed and promptly posts a good morning message on Facebook to his fellow “3 o’clockers.” Despite the early hour, dozens of Leominster residents, some working the third shift, greet him in the comments. “Good morning Mr. Dean!” “Mornin’ captain!” Before he was elected mayor of this sleepy city in North-Central Massachusetts, Mazzarella worked nights as a Leominster police officer. Decades …