OK- we have 2 bathrooms, one in hallway other in the back of the house but they are back to back. When the washer is going and or shower (in either bathroom) the toilet in the hall bathroom begins to bubble almost immediately. Eventually, the back bathroom will bubble as well. No problems with backup in showers or sinks and no trouble with the washer. The middle bathroom is VERY slow to go down after flushing. I called "plumber" just to ask what he thought the problem might be. He said the septic tank. My "gut" just won't let me think that is the cause. Any suggestions and thoughts on what I should do?

Bubbles indicate a partial blockage. What is happening is that the discharge ones down the drain pipe and hits a partial blockage. If that's bounces back sending a bubble of air ahead of it. Let checked the septic tank.
Locate and open the house clean out. Now flush the toilet a few times and what's the flow.
Is there any backups? Let me know. Back to you, Tom