Java adding to arraylist overrides other values

So I have this piece of code:

private ArrayList<Triangle3D> combine(ArrayList<Triangle2D> leftTriangles, ArrayList<Triangle2D> rightTriangles) {
    ArrayList<Triangle3D> returnTriangles = new ArrayList<Triangle3D>();
    for (Triangle2D eL : leftTriangles){
        Triangle2D eR = eL.getOtherTriangle(rightTriangles);
        if(eR != null){
            ArrayList<Point3d> corners = new ArrayList<Point3d>(3);
            for(Tuple<Integer, Integer> cornerL : eL.getCorners()){
                Tuple<Integer, Integer> cornerR = eR.getCorrespondingCorner(cornerL);
                if(cornerL != cornerR){
                    corners.add(addDistances(cornerL, cornerR));
                }
            }
            returnTriangles.add(new Triangle3D(corners, eL.getColor()));
        }
    }
    return returnTriangles;
}

But for some reason whenever i excecute the line:

        returnTriangles.add(new Triangle3D(corners, eL.getColor()));

The previous "corners" values of the Triangle3D elements that are already in the list get overriden by the newle added ones. Which I think is weird because I clearly make a new Arraylist on this line:

        ArrayList<Point3d> corners = new ArrayList<Point3d>(3);

I defined the Tuple class myself as following:

package vision.polyhedradetection;

public class Tuple<X, Y> {
    private final X x;
    private final Y y;
    public Tuple(X x, Y y) {
        this.x = x;
        this.y = y;
    }

    public X getX() { return x; }
    public Y getY() { return y; }

    public Tuple<X, Y> copy(){
        return new Tuple<X,Y>(this.x, this.y);
    }


    @Override
    public String toString() {
        return "(" + x + "," + y + ")";
    }

    @Override
    public boolean equals(Object other) {
        if (other == this) {
            return true;
        }

        if (!(other instanceof Tuple)){
            return false;
        }

        Tuple<Integer, Integer> other_ = (Tuple<Integer, Integer>) other;

        return other_.x == (this.x) && other_.y == (this.y);
    }

}

EDIT:

I found the root of the problem. But still don't know how to fix it. It's in my Triangle3D class. This is my class:

package vision.polyhedradetection;

import javax.vecmath.Point3d;
import java.util.List;

public class Triangle3D {
    private static Point3d corner1 = new Point3d();
    private static Point3d corner2 = new Point3d();
    private static Point3d corner3 = new Point3d();
    private int color;

    public Triangle3D(Point3d corner1, Point3d corner2, Point3d corner3, int color) {
        this.corner1 = corner1;
        this.corner2 = corner2;
        this.corner3 = corner3;
        this.color = color;
    }

    public Triangle3D(List<Point3d> corners, int color) {
        this.corner1 = corners.get(0);
        this.corner2 = corners.get(1);
        this.corner3 = corners.get(2);
        this.color = color;
    }

    private static Point3d centroid;

    public Triangle3D(Point3d centroid, int color) {
        this.centroid = centroid;
        this.color = color;
    }

    public Point3d[] getCorners() {
        Point3d[] Corners = {corner1, corner2, corner3};
        return Corners;
    }

    public int getColor() {
        return this.color;
    }

    public Point3d getCentroid() {
        if (this.centroid != null) return this.centroid;
        else {
            Double x = (double) Math.round((corner1.x + corner2.x + corner3.x) / 3);
            Double y = (double) Math.round((corner1.y + corner2.y + corner3.y) / 3);
            Double z = (double) Math.round((corner1.z + corner2.z + corner3.z) / 3);
            this.centroid = new Point3d(x, y, z);
            return this.centroid;
        }
    }
}

The problem lies in this code in my combine function:

        returnTriangles.add(new Triangle3D(corners, eL.getColor()));

For some reason when I create this new Triangle3D my corner1, corner2, corner3 in the "new" creating triangle are already set (thus they are pointing to my already existing corners). How do I get rid of this dependency? I don't get it since i make new corners when I create that class.