Okay, I'll take this context in mind and try to explain building a story.
Will have had simply means that in the future, you'll finish/have something in past! If that makes you confused, let me simplify.
Once I finish this pancake I will have had five pancakes.
It's 5 o'clock in the evening. And up till now you have already finished four pancakes. So, if someone asks you now, how many pancakes you had (up till 5 o'clock)? You'll simply reply - four.
Now, you cannot resist that mouthwatering dish and it's being baked. You'll then say... Once I finish this pancake (pointing at the oven, maybe at 1730 hr), I'll have had five pancakes.
I will - future (1730 hr); have had - go in future and come back as you already finished 4+1=5 pancakes (total pancakes till 1730 hr).
tl-dr: With perfect aspect, current/ongoing state is ambiguous. In your sentence, the time phrase may condition our view of the ongoing state.
I would interpret your original sentence as follows: at the point of retirement, you may or may not own one car, but you won’t concurrently own all three (in that case, you would have used ‘will possess’.)
Perfect aspect does not say anything about the current/ongoing state – that is the domain of the continuous aspect. Thus current/ongoing state is ambiguous, although we may infer something about it from the situation or other factors. For example,
I have owned a car.
It’s not clear if you currently own one, but we can infer that you don’t, because otherwise you would have just used the present simple. This is an inference, because the current state is ambiguous.
Different time phrases used along with perfect aspect may define (or imply) whether the action is ongoing or not. For example,
I have owned this car since 2011.
Clearly you still own it.
I have owned this car up to/until now.
Clearly you don't own it anymore.
It seems that with your original sentence, it’s the time phrase that conditions our view of the state of possession at the point of retirement.
Addendum: Concerning your idea about stative vs active verbs, I’m not sure if that’s relevant here. Look at the following examples.
By the time he retired, Alan had been married three times.
By the time he retired, Alan had gotten married three times.
Being married is a state, getting married is an action. But in neither case do we know if Alan is currently married.
Best Answer
No wonder you cannot get your head around it. As stated, the examples are mislabeled.
The future perfect is used to describe an action which will be completed at some time in the future.