In the transition from ArcGIS Pro 2.9 to Pro 3 the ArcGIS.Desktop.Mapping.LayerPosition was removed i.e.
see https://pro.arcgis.com/en/pro-app/3.0/sdk/api-reference/topic15120.html
https://pro.arcgis.com/en/pro-app/3.0/sdk/api-reference/topic15120.html#i-heading--detailed-list-of-...
look in API Changes Detailed List of Changes ArcGIS-Core 15th instance of 29 for LayerPosition.
The LayerPosition.AutoArrange option was very handy for adding external basemaps to a map. It would group basemaps together at the bottom of the Table Of Contents.
As in this useage:
layer = await QueuedTask.Run(() => LayerFactory.Instance.CreateLayer(uri, mapView.Map, LayerPosition.AddToTop, ShowName));
Alternative create an equivalent method or enum with the same functionality as the AutoArrage option.