a) security updates are delivered monthly by google.
thus, there were - afaik - also monthly updates by fairphone.
b) there also were minor bugfixes for FP3 in those updates .
c) for that, you have to ask fairphone support directly 
d) I didn’t found a page with release notes. But here you can see the current OS-version.
But on this forum you can find topics regarding those OS-updates
Like the january update: FP3 A.0110, Android9 OS update, patchlevel 5th, January 2020
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