final int SPEED = 3;
final int RADIUS = 10;

boolean left1 = true;
boolean left2 = true;
boolean left3 = true;

float x1 = random(50, width-50);
float x2 = random(50, width-50);
float x3 = random(50, width-50);

void setup () {
  size(200, 200);
}

// move balls
void moveballs() {
  if (x1+RADIUS>width) {
    left1=true;
  }
  if (x1-RADIUS<0) {
    left1=false;
  }
  if (left1) {
    x1-=SPEED;
  } else {
    x1+=SPEED;
  }
  if (x2+RADIUS>width) {
    left2=true;
  }
  if (x2-RADIUS<0) {
    left2=false;
  }
  if (left2) {
    x2-=SPEED;
  } else {
    x2+=SPEED;
  }
  if (x3+RADIUS>width) {
    left3=true;
  }
  if (x3-RADIUS<0) {
    left3=false;
  }
  if (left3) {
    x3-=SPEED;
  } else {
    x3+=SPEED;
  }
}

void draw() {
  background(50);
  fill(200, 50, 50);

  ellipse(x1, width*0.25, RADIUS*2, RADIUS*2);
  ellipse(x2, width*0.5, RADIUS*2, RADIUS*2);
  ellipse(x3, width*0.75, RADIUS*2, RADIUS*2);
  moveballs();
}