Q: A/C blowing hot air, filled 3 weeks ago with refrigerant and helped now refrigerant not cooling it.

Your car uses a refrigerant called R134a. If the re-charge 3 weeks ago resulted in your system working at that time, but a slow degradation in cooling effect has occurred since then, the most likely explanation is you have a leak somewhere in the system. The most common leaks are at compressor shaft deals and o-rings. YourMechanic can pressurize your system with nitrogen and then determine where the leak is using for example an infrared refrigerant leak detector and repair it.

Do NOT use stop leak products and even leak detecting dyes can be problematic UNLESS specifically approved by GM technical services. If you want a long term, professional fix, the best thing to do is find the leak and simply repair it, not mask over it with "stop leak". Once the leak is found and repaired you will need to replace your receiver/drier as part of the repair and add the required amount of refrigerant oil to the drier. Evacuate THOROUGHLY to remove all traces of moisture.

If you have YourMechanic diagnose the issue with your AC and do the job according to the instructions, you will be freezing when it is complete.