Answer
$\not \in$
Work Step by Step
The symbol $\in$ means that the number is an element of the set.
The symbol $\not\in$ means that the number is NOT an element of the set.
Since {$3$} is a set containing 3 as its only element. {$3$} is not one of the listed elements, then the missing symbol in the blank is $\not\in$
