Below are some of the changes which are made in Selenium 3 as compared to other versions:-
Selenium3, Selenium Core has been removed but supports Selenium RC however Selenium 2 is a combination of Selenium 1.0 (IDE) + RC.
Selenium 3 does not have HeadLess Driver. Whereas Selenium 2 has HeadLess Driver (HtmlWebDriver).
Selenium 2 is not part of W3C standards but Selenium 3 has become W3C (World Wide Web Consortium) standard.
Selenium 3 supports Firefox Browser version 47+. Whereas Selenium 2 support firefox version up to 47 and Require gecko driver for latest versions of Firefox.
If you want to learn Selenium you must take up the following Selenium Training Course. You can also watch the following Selenium video tutorial to learn all about Selenium.