3. The set of positive odd numbers $\{1,3,5, \cdots\}$ is grouped in ascending order such that the $n$-th group contains $(2 n-1)$ odd numbers: (first group) $\{1\}$, (second group) $\{3,5,7\}$, (third group) $\{9,11,13,15,17\}, \cdots$ Then 1991 is in the  group.