Alot of off the counter de cloggers specify if for metal or pvc. Things for metal are not for pvc and things for pvc are not suited for metal.
Personally, I would not use muratic acid in any drain, but.....
If you have to have this done every three years, then there is something more involved. Say, tree roots in your underground or under home pipes. You need to call a plumber with the capabilities to send a mini camera down your pipes and watch the line on a monitor as the camera passes the entire system.
This muratic acid in an accumulative state could have eroded the pipe under the ground and caused it to burst and tree roots and dirt has filled in.
Call a new plumber from the yellow pages that is a big company and have it re examined.
I just went through a total pipe refit and I have a home on a slab. This was expensive and tore up my lawn all over.