In most cases, they are interchangeable. Sometimes, depending on how it's used, you might have to give more information Example where it's interchangeable. Person A: "Are we at the petrol station yet?" Person B: "Pretty much/Almost (there)/Nearly (there)." Other uses: You can use 'pretty much' to confirm what someone has said. Person A: "So Emma told you she would come to the cafe after eating lunch?" Person B: "Pretty much" Using the same example, 'nearly' or 'almost' would be used if there's some parts incorrect. In this case, you would clarify. Person A: "So Emma told you she would come to the cafe after meeting Ben?" Person B: "Nearly/Almost. She's going to have lunch with Ben, then come to the cafe" If you would like more examples, I can try thinking up more