Seeking any advice on bad smell, PLEASE HELP!!!

This problem started about 3 years ago. I would get this foul odor while taking a shower. The toilet would not flush right. I called 3 different plumbers to check and fix the problem, but it keeps coming back. The last plumber ran a video thru the main line and used the *auger?* to clear the line. That worked for a year and it was wonderful. But now the horrible smell is back, the toilet is having a hard time just flushing water. Is there anything I can do??? PLEASE HELP MEEEEEEEEEE!

If you know the diameter and length of the main line you can check the flow rate and compare it with the expected value. In principle, at least.

The drain and vent system needs to be inspected. You could have a clogged vent or no vent where there should be one.

If the drain is clogging, it could be that it is not pitched properly. A plumber can send a camera down the line to inspect it for bellies, breaks, roots, etc.

Cacher_chick has the the best advice. Assuming (a dangerous thing to do) your plumber ran the auger all the way through the drain to the sewer main, it would indicate one of the possible problems listed exists. A camera inspection would be a wise next step. By all means do not use chemicals. They are dangerous and do not work anyway.