Selenium 3 is Selenium 2 without Selenium 1.
Other major changes is for the Firefox driver.  So far Selenium has been giving preferred treatment to Firefox, there was no need to use any browser drivers like for Google Chrome, IE, Opera, Safari (previously).  Firefox always ran right out of the box, not anymore…  We now need to use Geckodriver to run Firefox marionette implementation.
Other than this there are some major changes that you should be aware of as below:

  1. You’ll need to be running Java 8 to use the Java pieces of Selenium. Get it here
  2. Support for Firefox is via Mozilla’s geckodriver. Get it here
  3. Support for Safari is provided on macOS (Sierra or later) via Apple’s own safaridriver. Get it here
  4. Support for Edge is provided by Microsoft through their webdriver server. Get it here
  5. Only versions 9 or above of IE are supported. Earlier versions may work, but are no longer supported as Microsoft no longer supports them. Remember you need to have these settings for IE.