The common role of Old Fezziwig, Fred, and Bob Crachit is to serve as an example to Scrooge of a different way of living, acting, and relating to others.
Old Fezziwig was Scrooge's employer, when Scrooge was a young and inexperienced apprentice. He obviously had a prosperous business - he could afford to throw a lavish Christmas party for employees, friends, and family. At the same time, he obviously was a good mentor for Scrooge, who was able to build his own very profitable career from his start with Fezziwig. But what a contrast between Fezziwig's way of interacting with the young Scrooge and the older Scrooge's relationship with Bob Cratchit! Scrooge recognized the value of Fezziwig's actions, and realized how they applied to his situation.
He has the power to render us happy or unhappy; to make our service light or burdensome; a pleasure or a toil...I should like to be able to say a word or two to my clerk just now. That's all.
Bob Crachit and his family celebrated their Christmas "feast" with a goose that served them all but with no leftovers, "eked out by apple sauce and mashed potatoes" and a Christmas pudding that would have been considered small by anyone outside the family. However, they shared it all, reveled in the marvels of what they had, rejoiced in the time spent together, and loved each other deeply. Scrooge could not resist the warmth in the observations, and could not bear to hear the Spirit remind him of his comment; "If he be like to die, he had better do it, and decrease the surplus population."
Fred is determined to never allow his uncle's sour attitude to deprive him of the opportunity to attempt to share more positive times with family.
the consequence of his taking a dislike to us, and not making merry with us, is, as I think, that he loses some pleasant moments, which could do him no harm...I mean to give him the same chance every year, whether he likes it or not, for I pity him.
In observing all of these characters, Scrooge gains exposure to new ways of thinking about Christmas and responding to those meanings.