These three documents do different jobs.
The certificate of insurance confirms the personal and policy-specific details of the cover, such as who is insured and the dates of cover. The policy wording explains how the cover works in detail, including conditions, exclusions, claims rules, and definitions. The IPID, or Insurance Product Information Document, gives a shorter summary of the main insured events, exclusions, and restrictions.
If there is a conflict between a short summary and the formal contract documents, the formal policy documents matter more. That is why it is helpful to keep all three when they are available.