A little new, so bare with me, theoretically, when I am lifting a heavy load with the front bucket, and use my 3 point weight cement bucket to balance thectractor , am I taking away lift capacity from my loader?
No. The rear weight also helps remove front Axel load. The only way you could notice a strain is if you were lifting (raising not holding)rear weight at the same moment that you were operating front loader. Even then depending on the load and your particular tractor pump setup I doubt you would have a problem.
With weight hung off the 3PH you actually gain capacity. Without weight, most tractors have loaders strong enough to pick the back tires up well before the hydraulics are maxed out. Adding weight on the back allows you to lift stuff heavy enough to max the pressure out.