The rule with articles, as always, is that the definite article is used with something specific, and the indefinite article is used with something non-specific.
Although each person only has one life, different sections of a person's life marked by major changes may be referred to as different lives.
If you are speaking about a 'life' that is unique to a specific person, you might use the definite article. For example, a person who was a criminal and then reformed might refer to "the life I left behind". It is unique because it was theirs and is clearly definable in their past.
On the other hand, a person looking to make a life change might say "I want a new life for myself". This is because the new life has not yet been made, it is not a unique experience to them, and they may have many choices available to them, so it is not unique.
Your example 1 could work with either definite or indefinite articles, because "people" is non-specific. If you use the indefinite article, it refers to people in general; if you use the definite article you are singling out any individual within that generalisation and saying that each, in turn, have their own life.
Example 2 only really works with the possessive pronoun. If you speak personally about leaving "the life", it isn't clear which specific life you are talking about. I think even that "the life" is used idiomatically to refer to a life of crime, so if you said "I'm leaving the life" that could have a very specific meaning you don't intend.
Example 3 is really unrelated, because the article isn't associated with "life". You are right, 3a is correct.