when did three bars become the symbol for menu? thanks

[AllaN01Bear]

[TreasonousBastard]

1. Since we started thinking with our phones.

[RainCaster]

2. Yup

It's a mobile world

[AllBlue]

3. It's called a "hamburger menu"

"The inventor behind the hamburger menu icon is a man named Norm Cox. He made the burger icon for Xerox Star, which was the world’s first graphical user interface. That was more than three decades ago.

Afterwards though, the icon did a disappearing act.

It’s only relatively recently that the 3 line menu icon started creeping back, with the advent of mobile devices."

From : https://smallbiztrends.com/2015/01/3-line-menu-icon-hamburger-menu.html

[usonian]

4. Short answer: Like everything else, people created it because they could.

Not because it is any good. (pro's and con's below)

https://www.hostgator.com/blog/hamburger-menu-design/

What is the Hamburger Menu?
The hamburger menu is the common term used to describe the little three-lined icon used on many websites and apps to represent the larger menu. It most often shows up in the top left corner (but not exclusively), and clicking on it opens up the full menu.

5 pro's and 5 con's.

Hamburger or Not? Up to You
Every website is different, and deciding what will work for yours is up to you. Hamburger menus are one more design element to add to the possibilities for your site. They may help you do more with less in the space you have available, or you may determine they’re likely to be more confusing than they’re worth for your visitors.

Whatever choice you make, keep your visitors top of mind and make sure you’re building a website that will work for them.

My opinion? Who the hell makes a website that works for users?

I think it was Steve Jobs who said that making things SIMPLE is an extremely complex job.
That's why few actually do it.

edit: That article at smallbiztrends.com is really good.
RECOMMENDED.