How can I enable topological editing (create shared nodes on adjacent polygons or intersecting lines) and also activate the option to prevent polygon overlap (that when a new polygon is drawn, its portions that overlap on pre existing polygons is automatically removed) in QGIS version 3? In prior versions these setting were found in Settings->Snapping Options menu.
In QGIS 3.4 you may need to enable Snapping toolbar by right-click the main menu bar and select Snapping toolbar, and you will find this menu:
The last three icons are Enable Topological Editing, Enable Snapping on Intersection and Enable Tracing, respectively.
Also, in QGIS 3.4 there is a new feature that you can enable Topological Check during digitizing using Layer properties -> Digitizing -> Topological check:
You can refer to the changelog of QGIS 3.4 and scroll down to digitizing new features for more information: