This has been bothering for a while. Type 3 Immortality is defined as; Immortality via regeneration: Characters with this type of immortality can simply regenerate from wounds that would normally be lethal, though its effectiveness depends on the degree of the regeneration.
However, this doesn't explain what level of regen is needed for it to be Type 3. I doubt most people would consider, say, Low regen to be Type 3, but when is the line drawn?