3. In a convex polygon, a quadrilateral has two diagonals, a pentagon has 5 diagonals. Observe and explore how many diagonals a convex decagon has ( ).
(A) 29
(B) 32
(C) 35
(D) 38
3.C.

Draw and observe to explore the number of diagonals in polygons: for quadrilateral, pentagon, hexagon, heptagon, octagon, nonagon, the number of diagonals are: $\begin{array}{lllllll}2 & 5 & 9 & 14 & 20 & 27 & 35\end{array}$ The pattern is: $2+35+49+514+620+727+8$