You need to use a weight on the rear of the 3 point. If you stop to figure the amount of weight that you are adding to the front axle when used as shown you will quickly realize that you are overloading the axle. The rear weight will transfer some of the weight to the much stronger rear axle.
Yes it will pick up the load but at what cost? Weighted tires and wheel weights improve the stability of the tractor but they do nothing to reduce the strain on the front axle.
First off, you'd need to know the weight distribution on each axle without any additional load i.e. tractor + fel + front implement + liquid ballast + wheel weights + fuel before you can say anything meaningful.
Taking what I know about the weight of the tractor, the loader, the ballast and the wheel weights, and if I make the massive assumption that the weight distribution on each axle is 50/50 and assume that the disc harrow weighs 1,500lb, and then I sum the moments about each axle, I get the following:
The weight on the front axle without a rear ballast box or implement is 7,515lb. With an 800lb ballast box with the center of mass located 4ft behind the rear axle, the load on the front axle drops to 7,058lb.
So am I overloading the front axle? Without scales I have no idea what the weight distribution of the tractor is. With over 2,000lb added to the rear axle in the form of liquid ballast and wheel weights it might be 60% rear and 40% front which would change the numbers I mentioned above.