Universal set is the set which contains all the elements of the other sets.
Let A = {1, 2, 3, 4} , B = {3, 4, 5, 6}, C = {6, 7, 8}
So, universal set will have all the elements of set A, B, C
∴ U = {1, 2, 3, 4, 5, 6, 7, 8}
Note that:
All the sets are subsets of Universal set
A ⊂ U
B ⊂ U
C ⊂ U