A relation is transitive if whenever $x\sim y$ and $y\sim z$ then $x\sim z$.
Notice however, that if the relation is transitive and $x\sim y$, $y\sim z$ and $z\sim w$ as well, this further implies that $x\sim z$ and $z\sim w$ which implies that $x\sim w$.
When finding the transitive closure of a relation, you need to add all ordered pairs $(x,y)$ where you can write a sequence of pairs that already exist in the relation $(x,a_1),(a_1,a_2),\dots,(a_{n-1},a_n),(a_n,y)$ regardless of length.
In the relation above, you can write $2\mapsto 0\mapsto 3$, so you will need to add the pair $(2,3)$. You can also travel $2\mapsto 0\mapsto 3\mapsto 2$, so the pair $(2,2)$ will also need to be added.
Similarly for the others that you already found and $(3,3)$. However, the pair $(1,1)$ is not going to be added as there exist no pairs where $1$ is the second entry (nothing leads to it). As such, it doesn't cause any problem for the transitivity of the relation.