Are HTML5, CSS3 AND JS enough to make a fully functional web page?

Answer 1

Yes with just front end functionality.

Answer 2

Yes

Answer 3

You are right.
But beleif me if you once use bootstrap and materialize after these , your work of front end will become super easy.
You can complete a responsive page in 30 mins hardly.
I recommend you to check one of them after html css and js.

They are based on these three but what have to do is just copy paste code for almost component.you can also costomize things according to your need there.

Once you will complete one and practise a webpage using it.you will forget about these for web page.

You can check for bootstrap and materialize rough codes in my codebit .and if serious visit
Tutorialspoint or w3school.