Why QGIS 3?
Free, multiplatform, extensible.
Notable Missing Features from QGIS 2
No built-in OSM download tool
It was a very good feature to get you started by adding maps data to your project.
Some extension still only for QGIS 2
Convert your linear data first
Large GeoJSON, CSV, KML should not be used directly. You need to convert to indexed datatypes first (i.e. GPKG, SHP) for better performance. This will be visible when you try to perform points clustering.
JSON compatibility issue