In Reallusion Content Marketplace or Google 3D Warehouse, there are a lot of content such as props, background models. You may export these models into Unity3D via 3DXchange5.
Go to "File" >> "Export to other 3D format" >> "FBX". Select preset as "Game Engine - Unity3D" in FBX export window.
Use the Project View in Unity. Right-click to import FBX exported from 3DXchange5 and move the file into your right folder. You can manage models as Assets in Unity3D.
Drag Assets into the 3D view or Hierarchy view to build your scene in Unity3D.