Your negative sentence is in the past tense, Melanie, where the third person singular inflection does not appear. He doesn't have three cars'-- it is the operator ('do' here) which takes the '-s' inflection.
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Anonymouswhich is correct use have instead of hasYour question is very broad.
Which is right?
He has a car, or
He have a car.
The sentence formation is not a question statement.
It is an information for some one to say.