Which is right — "all the three" or "all three" in a situation of specific objects?

Let's imagine, I'm speaking about someone's three specific cars, which are over there, there, and there. Now I'm saying that I have installed a new sound system in all of them. Which is right:

• [...] I have installed a new sound system in all the three cars.
• [...] I have installed a new sound system in all three cars.

Solution 1:

All three cars is idiomatic. All the three cars is perfectly grammatical: to me it has an implication that you have already somehow designated three particular cars from a larger set. This possibility is not implied by all three cars (though it's not excluded by it either).

Solution 2:

As TrevorD says, all three cars implies that there are only three cars. Three of the cars would be the best option if you are referring to three out of several cars. All the three cars or all of the three cars are both grammatically correct, but sound cumbersome. All three cars already implies that you are referring to specific cars; you don't need the article to clarify that these aren't just any cars.

I would say that your best options are all three cars, three of the cars, or each of [the] three cars, depending on the context.