Damaging someone else's property is not protected free speech in the US. Nor is using someone else's property without permission and against the owner's wishes.
Scenario 3 would surely be vandalism or "Malicious Mischief" or some similar offense, even if the paint can be fairly easily removed. The same would be petty surely true of scenario 2, as it would take at least some effort to remove the sticker, and it could be a safety hazard until it is removed (obstructed vision).
Scenario 1 would probably not be even a minor crime, unless perhaps the person trespassed to attach the string. But the person has no right to insist that the car's owner not remove the sign.
I am assuming that the car is owned by someone else, and the the person placing the sign, sticker or paint acted without permission from the owner or any authorized person.