A good strategy is to pick the biggest one and boycott the he'll out of it.
It's doesn't matter how, go with number 2, go with number 3, anything but number 1.
If enough people do that, it would destroy 1 or completely eliminate it. Then once it fails, move the boycott onto the new number 1 and repeat.
We can't do much by going after all the big three because we all need to use someone's services. Some people have choice but most of us who don't live near a major city have no choice. So the only logical way to create an effective boycott is to target one company and put all our efforts into that.
Once the first boycott is successful, the next corporation will have to think about how to handle the situation. It makes an example of what is possible with organized boycott.