Circle object of v3 creates a circular overlay as expected. It is more useful than I first thought.
If you want to zoom your map to show an area of say 50 km radius, that would mean some maths. Create a Circle() of 50 km radius and take its bounds by now provided getBounds(). You can use the bounds to zoom the map by fitBounds(). No calculations needed. An example.
Circle can be used as a marker as well. It is clickable and it works as an anchor for InfoWindow. Marker keeps its size when you zoom. Circle does not but Circle has a minimum size based on stroke width. That is actually nice behavior in many cases. Now there is an updated version of makeMarker(). It creates a Circle instead of Marker if you set any value to 'radius' option.
'title' option does not create a flyover tooltip. However Circle supports all the usual mouse events. So you can create a custom tooltip for the purpose.