If you got too many mesh in your imported files and you want to put the same material (3D Renderer) over all of them, you can Group their generators as followed :
First select all the mesh you want to group and do a right click:


Then choose a 3D Renderer among the list.


And there you go, no a single material is applied on the overall geometries. Note that in this status, 3D Modifier can also be applied on many different mesh altogether: