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 to stand for the number of elements is set then
.
That is called the inclusion-exclusion principle.
Do a web search on that topic. There are many generalizations of that idea.