If you already don't have contact with these three firms because you think they are terrible or they just aren't relevant to you? You can help by telling people about the boycott.
The somewhat mild reputation of the NAACP works in our favor on this one. They have a pretty positive reputation even among moderates. Takes a lot to make them angry.
"Did you know the NAACP wants a big boycott of McDonald's, Walmart and Meta?"
Make it common knowledge.