3. Arrange the positive integers $1,2, \cdots, n, \cdots$ into groups where the $k$-th group contains $k+1$ numbers: $(1,2),(3,4,5),(6,7,8,9), \cdots$, then 2016 is in the $\qquad$ group.
$$
2+3+4+\cdots+(n+1)=\frac{n(n+3)}{2} \geqslant 2016 \Rightarrow n(n+3) \geqslant 4032 \Rightarrow n \geqslant 63 \text {. }
$$

So 2016 is in the 63rd group.