The rule of thumb is two to four pounds of refrigerant per ton of your machine. Most home air conditioning units are between one ton and five tons. So, with that in mind if you have a three ton unit then you would need between six pounds of refrigerant upwards to twelve pounds of refrigerant.
It is important to note that these are rough estimates and the exact measurement of refrigerant that your unit needs varies. It is always best practice to review the exact specifications on your unit to see exactly what you need.