A new idea to elucidate quantum reaction dynamics of heavy-light-heavy (HLH) systems is proposed on the basis of the hyperspherical elliptic coordinate approach. This coordinate system has a big advantage of nicely expressing good vibrational adiabaticity in the HLH systems. Taking this advantage, the concept of potential ridge is introduced, for the first time, in three-dimensional reactions. The potential ridge is proved to be very useful to extract some important avoided crossings which dominate the reaction dynamics, in fact, qualitative features of the reaction dynamics can be interpreted in terms of nonadiabatic transitions at those important avoided crossings near the potential ridge. Examples are: (i) onset of reaction for a specified initial rotational stale, and (ii) major reactive transition for a specified initial rotational state. Avoided crossings to the left of the potential ridge are also useful to interpret certain aspects of reactions accompanying vibrational transitions. The new idea mentioned above is applied to atypical HLH reaction O(P-3)+HCl-->OH+Cl with the use of two types of potential energy surfaces.