UUID v3 is a name-based universally unique identifier that uses MD5 hashing to generate deterministic UUIDs from a namespace and name combination. Like UUID v5, UUID v3 produces the same identifier every time you use the same namespace and name inputs, making it suitable for creating consistent identifiers in legacy systems and applications requiring backward compatibility.

The UUID v3 specification follows RFC 4122 standards and generates 128-bit identifiers in the standard format: xxxxxxxx-xxxx-3xxx-yxxx-xxxxxxxxxxxx, where the '3' indicates version 3. While UUID v5 (SHA-1) is now preferred for new applications, UUID v3 remains important for maintaining compatibility with existing systems that use MD5-based UUID generation.

UUID v3 should be used primarily for backward compatibility with legacy systems that already use MD5-based UUID generation. If you're maintaining an existing application that generates UUID v3 identifiers, continue using v3 to ensure consistency. However, for new projects, UUID v5 (SHA-1) is recommended as it provides better collision resistance.

Common use cases include legacy database migrations where existing UUID v3 identifiers must be regenerated, maintaining compatibility with older APIs or systems, working with third-party systems that specifically require UUID v3, or supporting legacy protocols and standards that mandate MD5-based UUIDs.

UUID v3 generation follows a specific algorithm: The namespace UUID is converted to its 16-byte binary representation. The name string is encoded as UTF-8 bytes. These are concatenated and hashed using MD5, producing a 128-bit hash. This entire hash becomes the UUID, with specific version and variant bits set according to RFC 4122 standards.

The version bits (4 bits set to '0011' for version 3) are placed in the time_hi_and_version field. The variant bits (2 bits set to '10') are placed in the clock_seq_hi_and_reserved field. This ensures the resulting UUID is RFC 4122 compliant and universally recognizable as a version 3 UUID.

UUID v3 uses MD5 hashing, which is considered cryptographically weak in modern security contexts. MD5 has known collision vulnerabilities where different inputs can produce the same hash. However, for UUID generation purposes where the goal is reproducible identifiers rather than cryptographic security, UUID v3 is acceptable for legacy compatibility.

Never use UUID v3 for security-sensitive applications, password hashing, or cryptographic purposes. The deterministic nature combined with MD5 weaknesses makes it unsuitable for security tokens or authentication. For new projects requiring deterministic UUIDs, always prefer UUID v5 (SHA-1) which offers better collision resistance while maintaining the same deterministic properties.