Explanation
Using a Connected App for each external system integration is a good solution because it can provide security, auditing, and monitoring features for each integration. A Connected App is an application that can connect to Salesforce using APIs and OAuth as an authentication protocol. A Connected App can also enforce policies such as IP restrictions, login hours, and session timeout for each integration. Using a shared integration user for the three external system integrations is not a good solution because it violates the principle of least privilege, as well as makes it difficult to audit the activities of each system. Using a shared Connected App for the three external system integrations is also not a good solution because it does not allow for granular control and visibility of each integration. Using a unique integration user for each external system integration is not enough to meet the security and auditing requirements, as it does not provide any mechanism for authentication, authorization, or encryption. Reference: Salesforce Integration Architecture Designer Resource Guide, page 20-21