When you start taking rings off, the kiln will have a harder time reaching temperature. With 3 rings they work together to deal with the heat loss from the lid and floor. With one or two rings they have to work harder and may not have enough power to deal with it. Two rings may work, but may not. Firing is pretty inexpensive, so don't feel too bad about firing it half empty.
Just wanted to say Neil, that your comment has been freeing for me. " Firing is pretty inexpensive, so don't feel too bad about firing it half empty." Since I stopped worrying about it, I have successfully made and tested 3 new glazes which I am so excited about. Because of others' suggestions, I am using cones in each firing which I date, and I have a book to record the firings and any nuances. Thanks.