(3) For the tetrahedron $A-BCD$, the projection of vertex $A$ onto the base $BCD$ is point $O$, and the distances from point $O$ to the three side faces are equal. Then point $O$ must be the ( ) of $\triangle BCD$.
(A) Centroid
(B) Orthocenter
(C) Incenter
(D) Circumcenter