Have had my crown replaced three times due to its bad odor. Recently had a deep cleaning and it smells worse. What's going on?

Your best bet is to check the fit of your new porcelain crown with an expert dentist near you as a second opinion.If your crown is over a root canal tooth, it is highly advisable to check the old root canal also, that might cause halitosis also.More clinical exam and xray is necessary to solve your problem, and explore the adjacent teeth next to your new crown.

Hello!!!! 
There are several factors that can cause a bad odor or taste when it comes to a crown some of them are the material of your crown, the tendency of modern dentistry is to eliminate the metals inside the mouth, metal free restorations decrease this bad characteristics other possibility are filtration this means that the crown is not sealed properly or fitted correctly or a bad hygiene and even a combination of both.
Other possibly not related to that crown or your oral health in general are digestive problems.

I hope my response was helpful!  

Have a wonderful day!