int r = 100;
void setup(){
size(500,500);  
}
void draw(){ 

r = r + 1;
fill(255, 0, 0);
ellipse(250,150,r, r);

fill(0, 255, 0);
ellipse(250, 250, r, r);

fill(0, 0, 255);
ellipse(250, 350, r, r);

}