
A union B union C
I need the formula to get the union of A,B,C. For Example. You have set A, set B and Set C. You have all of the information provided to you, other than what the union of all three is.
I have this formula(since i dont know how to input math symbols. int. means intersection):
aUbUc = (a + b + c)  (a int c)  (a int b)  (b int c) + (a int b int c)
would that formula work?

If we use $\displaystyle \left A \right$ to stand for the number of elements is set $\displaystyle A$ then
$\displaystyle \left {A \cup B \cup C} \right = \left A \right + \left B \right + \left C \right  \left {A \cap B} \right  \left {B \cap C} \right  \left {A \cap C} \right + \left {A \cap B \cap C} \right$.
That is called the inclusionexclusion principle.
Do a web search on that topic. There are many generalizations of that idea.