The notion of a third sensor did come up. However, it is not the role of the regulator per se to impose the design solution on the applicant. This is how the certification process works. The applicant will propose a design solution, and it's the regulator's job to ascertain whether that solution meets the standard. Design changes were implemented on the 737 MAX, which were found to meet the safety standards and to mitigate the specific concerns that were discovered in the post-accident investigation, without the addition of a third source.
You are correct that other aircraft do indeed have three sources, but that is not the only way that the system can be designed. We evaluated the aircraft with the existing number of sensors and the improvements that were made, and we found them acceptable.