Wow
Now, I moved the variables like int savedTime = millis();
already in the class because time shall be different for each triangle.
What to do
Now the class represents one triangle. ONE.
So display() should be brief: triangle(x, y, x+25, y+50, x-25, y+50);
(the for loop already is in draw() so NOT needed here )
Also you forgot to pass different values for x,y in setup() (because the false for-loop in display() took care of this). So in setup() calculate some values based on i.
Then use background(220); at start of draw() (NOT background(0); because black on black won’t work, you know… )
Now, go and fix your timer. 15 seconds is quite long though.
Got it working here.