See at Scratch 3: Add a button to 3.0 on project pages

SeeAtScratch3.js

// ==UserScript==
// @name         See at Scratch 3
// @version      1.0
// @description  Open Scratch 3 as fast as possible
// @author       apple502j
// @match        https://scratch.mit.edu/projects/*
// ==/UserScript==

function seeAtScratch3(e){
    if (document.URL.indexOf("#editor") > -1) return;
    if (document.URL.indexOf("#fullscreen") > -1) return;
    var btn = "<a href='https://llk.github.io/scratch-gui/develop/#{PID}'><div style='font-weight:bold;font-size:20px;background-color:#4baa29;color:white;display:inline-block;padding:8px;text-align:center;width:150px;height:20px;'>Open at 3.0</div></a>";
    var pid = document.URL.replace(/[^0-9]/g, "");
    btn = btn.replace("{PID}", pid);
    document.getElementsByClassName("buttons")[0].innerHTML = btn + document.getElementsByClassName("buttons")[0].innerHTML;
}

window.addEventListener("load", seeAtScratch3);