At this point, the Office 365 service only allows for the retention of audit entries for 90 days. Can you provide us more information regarding your requirement to keep logs for 3 years. Is this a legal obligation? Please provide details around the specific audit entries you would like to retain for an extended period of time.
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O365 Audit logs are now available beyond 90 days for E5 customers. This feature is in Preview. More details here - https://docs.microsoft.com/en-us/office365/securitycompliance/search-the-audit-log-in-security-and-compliance#before-you-begin
"The one-year retention period for audit records for E5 organizations (or E3 organizations that have Advanced Compliance add-on licenses) is currently available only as part of a private preview program. To enroll in this preview program, please file a request with Microsoft Support and include the following as the description of what you need help with: "Long-term Office 365 audit log private preview".