3. For any $r$-element subset $A$ of $X_{n}=\{1,2, \cdots, n\}$, let the smallest number in $A$ be $t(A)$. Find $\sum t(A)$.
(Let $t(A)=i$. Consider the number of $r$-element subsets with $i$ as the smallest element, obtaining $\sum_{t=1} t(A)=\mathrm{C}_{n+1}^{r+1}$. )