The Original Poster's four examples are all grammatical. However, the sentences with there are preferable in most situations. The reason is that the phrases a car and three cars are indefinite. A car uses the indefinite article a, not the. The phrase three cars doesn't use the word the, as in the three cars. It's also therefore indefinite. This means that we haven't been speaking about the car or the three cars before.
In English we don't like to use indefinite noun phrases as Subjects. We like to put old information, stuff that we have already talked about, at the beginning of the sentence. If we put indefinite noun phrases at the beginning of the sentence, the sentence is more difficult for listeners to process.
If a sentence uses the verb BE, we can avoid this problem by using an existential sentence. We can use there is or there are. This moves the new information to later in the sentence where it is easier for the listener to process. Look at the following groups of sentences:
- A cafe is on the corner.
- A riot was in the West End.
- An accident was on the motorway.
- A meeting is at 2 o'clock in the main hall.
- A God is in heaven.
The sentences above are all grammatical, but they are strange and awkward. There may be some special time when want to say these sentences like this, but usually they are not a natural way of phrasing the sentence. If you generally speak like this, people will find it difficult to understand you, and they will know that your English is not very good.
If we use an existential construction instead (there is or there are), the sentences become completely natural:
- There's a cafe on the corner.
- There was a riot in the West End.
- There was an accident on the motorway.
- There is a meeting at 2 o'clock in the main hall.
- There's a God in heaven.
Notice that the problem in the first group of sentences is that we used indefinite noun phrases. If we use definite noun phrases, the sentences become completely natural. Look at these versions of the sentences:
- The cafe is on the corner.
- The riot was in the West End.
- The accident was on the motorway.
- The meeting is at 2 o'clock in the main hall.
- God is in heaven.
Those sentences sound like native-speaker sentences.
All four sentences are grammatically well-formed. However, the sentences with there are more natural, and will make you sound like a better English speaker. They will also make it easier for the person listening to you.