3. Given the sequence $\frac{1}{1}, \frac{2}{1}, \frac{1}{2}, \frac{3}{1}, \frac{2}{2}, \frac{1}{3}, \frac{4}{1}, \frac{3}{2}, \frac{2}{3}$, $\frac{1}{4}, \cdots$. Then, $\frac{n}{m}$ is the nth term of this sequence?
(Answer: the $\frac{(m+n-1)(m+n)}{2}-n+1$ term)