The square that shows all running apps became a vertical hamburger in recent Android versions. I hate those endless icon updates, usually to something less intuitive than before. The gallery icon became a huge asterisk (or is that a stylized flower?), and the distinct camera icon became something that looks like a kid's rubber toy.
The triangle (points to the left?) is an "improved" backarrow.
Hartmut
I have a triangle, circle, and square. The square shows all running apps