(The Triangle class) Design a class named Triangle that extends GeometricObject

You need to create a new Triangle object like this, so that you have a reference

Trangle triangle = new Triangle(side1, side2, side3);

  //     ^^^^^^ This is the most important thing you're missing.  You need a reference
  //            point for your object.  That's the only way you can access it's
  //            properties.    

You also need to set it’s filled and color properties

triangle.setFilled(filled);
triangle.setColor(color);

Then, you can invoke its methods like this:

System.out.println("The Triangle Sides are \n side 1: " 
       + triangle.getSide1() + "\n Side 2: " + triangle.getSide2() 
       + "\n Side 3: " + triangle.getSide3());

System.out.println("The Triangle's Area is " + triangle.getArea());

System.out.println("The Triangle's Perimeter is " + triangle.getPerimeter();

System.out.println("The Triangle's Color is " + triangle.getColor());
System.out.println("Is the Triangle filled? " + triangle.isFilled());

You’re able to access the GeometricObject‘s isFilled(), setFilled(), getColor(), and setColor() because a Triangle is a GeometricObject (extends), so it inherits all its methods.

By the way, this is not how to calculate the area of a triangle:

public double getArea() {
    return (side1 + side2 + side3) / 2;   // This is so wrong
}

Check out this link for correct formula

Edit: With another problem with code

public double setSide1() {
    return side1;
}

public double setSide2() {
    return side2;
}

public double setSide3() {
    return side3;
}

/**** Should Be ******/

public double getSide1() {
    return side1;
}

public double getSide2() {
    return side2;
}

public double getSide3() {
    return side3;
}

Edit: Triangle Formula

public double getArea() {
    int p = getPerimeter() / 2
    return Math.sqrt(p * ((p - side1) * (p - side2) * (p - side3));
}