hello and welcome to HCM forum,
I would like to inform you that if the root canal treatment has been completed with a permanent filling on the tooth then pus discharge is not expected but if your dentist has placed a temporary medication in your tooth then it might take a few days for the pus discharge to stop and infection to resolve completely.
I will advise you to take a second opinion and visit an endodontist.
For now, you can use a mouth wash twice/day as well as perform warm saline rinses 3-4 times/day.
The dentist might have to prescribe you antibiotics to resolve the infcetion which still persists.
I hope this answer was useful,
I wish you good health,
take care.

Hi,

Thanks for asking the query,

According to your clinical symptoms i suspect that you have developed severe infection in your root canal treated tooth.
I would suggest you to visit to the Dentist and get the checkup done, take an x-ray of the affected tooth.
Start with re-RCT with the tooth.
Start with the course of antibiotics and analgesics under the prescription of your Dentist.
Go for complete mouth scaling and polishing.
At home take lukewarm saline and antiseptic mouthwash rinses.

Hope this helps out
Regards.

Welcome to HCM forum. As per your history, It is extremely unusual to have pus discharge from a root canal treated tooth after three weeks of completion of treatment. Your bad odor and bad taste are due to collection and discharge of pus into your oral cavity. If you do not have other gum disorder like periodontitis then it is almost obvious that your treated tooth root is still harboring some bacteria as PROBABLY it is not properly sealed. Only examination of 'tooth and gum' by your dentist will not disclose the reason behind it. You may need a couple of good IOPA radiograph of the affected tooth from different angle to see the state of root canal sealing and condition of tooth apex area. Listerine mouth wash will give you a temporary relief from your bad odor and taking antibiotics also may not help as it will not able to control the bacteria, which may still present into the unfilled spaces into your root canal. YOU SHOULD IMMEDIATELY CONSULT A QUALIFIED ENDODONTIST FOR FURTHER INVESTIGATION to reach to an actual diagnosis and management.
Wishing you a good health.