First, I am so sorry for your loss and applaud you taking on the care of three children. They are incredibly lucky.
Second, if you're not religious, don't "blame" it on God- just my thought.
I would be simple, but honest.
"Jack, your mommy and daddy died in a car accident because someone wasn't paying attention while they were driving (or a short version of whatever happened). They didn't do anything wrong and they loved you very very much. I am so lucky that they trusted me to take care of you."
Don't use metaphors, he's too young. Answer each question as it comes up using very true and literal terms. Remember, the car didn't cause the accident (unless is was faulty)- the driver did.
If you are really struggling with this one, schedule an appointment with a children's therapist and get some specific suggestions!