Find the number of elements in $A_1 \cup A_2 \cup A_3$ if there are 105 elements in $A_1,$ 1004 elements in $A_2$ and 10018 elements in $A_3$ in each of the following situations:

(a)   The sets are pairwise disjoint.

(b)  $A_1 \subseteq A_2 \subseteq A_3$.

(c)  There are 18 elements common to each pair of sets and 5 elements in all three sets.

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