What are three specific behaviors of the Finch family that Aunt Alexandra disapproves of in To Kill a Mockingbird?
As you pointed out, Aunt Alexandra has serious problems with the way that Scout dresses. Scout says that, in fact, "Aunt Alexandra was fanatical on the subject of [her] attire.
But she is also very concerned about the way that Scout speaks, particularly since she tends to swear when she gets very excited or angry. She also finds herself upset when she sees Scout scratch her head, this is not lady-like and therefore certainly not something to be done when anyone else can see.
Another has to do with the way that Scout does not take on airs the way that Aunt Alexandra believes that she should. Aunt Alexandra believes quite strongly that the Finches are somewhat elevated above other people in Maycomb and as such should act differently. Running around playing in her boyish clothes is just not an acceptable way for a lady of Scout's background to behave.
Along those lines, one more specific behavior that Aunt Alexandra does not approve of is Scout going to Calpurnia's church or her home. She simply does not think it is acceptable for Scout to go to a church where the congregation is black or to the home of a black person, no matter how beloved to the family.
As was mentioned in the previous post, Aunt Alexandra disapproves of Scout's appearance and behavior. She wishes that Scout would act more like a lady by wearing dresses and socializing with other females. Alexandra also disapproves of Atticus' decision to defend Tom Robinson. Alexandra is prejudiced against African Americans and poor white families, which is why she forbids Scout from playing with Walter Cunningham Jr.
According to Scout, Aunt Alexandra also has a "preoccupation with heredity" and cares deeply about her family's background. When Aunt Alexandra moves into Atticus' home, she argues with him about teaching their rich family history to Jem and Scout. She finds it displeasing that Atticus has not instilled in his children a sense of familial pride. She even convinces Atticus to have a discussion with Jem and Scout about the importance of their family name throughout Maycomb County. Aunt Alexandra wishes that Scout and Jem would behave like respectable individuals from a prestigious family.