You've done everything "by the book", except that you draw your triangle off-center. Think of it as your triangle should be drawn in relation to the origin of the matrix (0,0). Not the center of the sketch, when you use "translate" (with push/pop matrix) you basically reset the origin.

Edit: Perhaps I should also have pointed out that rotate doesn't rotate the triangle, it rotates the matrix around it's origin. Which is the reason why we (usually) need to draw everything that should rotate relative to the origin of the matrix, which is (0,0).

Try this, note that I haven't taken any care to actually make a lovely equilateral triangle so the center will be a little bit "off". I just made it "sort of even" to give you a hint of how you should draw your triangle.

1. pushMatrix();
2. translate(width/2,height/2);
3. rotate(s*radians(90));
4. triangle(-30, 30, 0, -30, 30, 30); 
5. popMatrix();

Thank you for the response. I'm still sort of confused though. I don't want the triangle to be centered (since I have three of them next to each other), but when I change the values of the triangle again, the rotation is off again. I'm clearly misunderstanding something.

You can add an additional translate() after the rotate() to reposition the triangle after rotation.
For example:

1. void setup()
2. {
3.   size(200, 200);
4. }
5. 
   
6. void draw()
7. {
8.   background(255);
9. 
   
10.   pushMatrix();
11.   translate(width/2,height/2);
12.   rotate(frameCount*radians(90) / 20);
13.   translate(0, -60);
14.   triangle(-30, 30, 0, -30, 30, 30);
15.   popMatrix();
16. }

I never thought of that. Thank you!