Once you've installed the Growth-X Chrome Extension, and logged in using your GX credentials, you'll be able to automatically uninvite people that haven't accepted your invitation to connect.
Step 1: Navigate to your LinkedIn pending invites list.
Step 2: Search for leads that that were invited 3 or more weeks ago. Those are the people that you will want to uninvite.
Step 3: Take a note of which page the 3-week old leads appear, click the GX extension icon. Place the page-number into the Chrome Extension's pop-up, and press the "Withdraw" button.
The app will do the rest.