#2 is not grammatically correct, and it's also completely nonsensical. The other two are grammatically correct, but almost certainly don't mean what you're trying to say. #1 could only make sense if you're a talking car. #3 could only make sense if you're a traffic cop or a parking attendant.

I'm not sure that any of them are correct because it's not clear what is meant by them. 1. What do you want to express? How can 'we' be 'all those cars'? Do you mean that we are IN all those cars? 2. This isn't correct. 3. This could mean that you have caused the cars to be stopped. However, it's odd and doesn't make sense that it's present tense. 'All those cars' suggests a specific group of cars whereas present tense suggests this is generally or regularly true.

Did you mean to say “why” rather than “we”?

I would say sentences 1 and 3 are correct, while 2 is not. 1. We are all those cars stopped at the corner. This is grammatically correct. The sentence implies that "we" is equivalent to "all those cars stopped at the corner." It's a metaphorical or poetic way of saying that you might be the car in the movie 'Cars'. 2. We are stopped all those cars at the corner. This is grammatically incorrect. The structure is problematic. "We are stopped" is in passive voice, but "all those cars at the corner" doesn't fit well with it. 3. We have all those cars stopped at the corner. This is also correct. The sentence either implies that "we" have caused or are responsible for "all those cars" being stopped at the corner or '"We" own or possess "all those cars" that are currently stopped at the corner. I hope that helps!