Answer 1

The main purpose of learning them? So you know how to use them. Anyways, why not just look up each one and read about it so you know what they are or what their differences are? Trust me, most people here, including myself, make for a bad organic version of Google.

Resources for you to research:

https://www.sololearn.com/Course/CPlusPlus/
https://en.wikipedia.org/wiki/C%2B%2B

https://www.sololearn.com/Course/Java/
https://en.wikipedia.org/wiki/Java_(programming_language)

https://www.sololearn.com/Course/JavaScript/
https://en.wikipedia.org/wiki/JavaScript

https://www.educba.com/c-plus-plus-vs-java/
https://en.wikipedia.org/wiki/Comparison_of_Java_and_C%2B%2B
https://www.upwork.com/hiring/development/java-vs-c-which-language-is-right-for-your-software-project/
http://www.computing.dcu.ie/~renaat/projects/cvjava.html
https://www.javatpoint.com/cpp-vs-java
https://www.quora.com/What-are-major-differences-between-C++-and-Java

https://www.quora.com/Web-Development-How-is-JavaScript-different-from-languages-like-C++-and-Java
https://techbeacon.com/javascript-vs-other-languages
http://www.differencebetween.info/difference-between-javascript-and-cplusplus

^There you go. That's more than enough to educate you on what you're asking. If you type your exact question into Google, it returns 43 MILLION results for it, which is a much better response than you'll ever get here. :D Best of luck to you.