Welcome to the Forum! 
First of all, here you find all Releases for the Fairphone 3 and the Fairphone 4.
It is important to note that Fairphone does not guarantee monthly updates. As I saw though, the Fairphone 4 was on a nearly monthly schedule and the Fairphone 3 gets updates usually every 2 months. However, the Android patch level can be up to one month behind at a release. It is best to see for yourself. The threads are quite detailed.