merge objects for 3d printing I have several objects (see the attached picture) I want to merge, so that I finally get one uniformly mesh.

In the end I want to 3d print this object but the slicer software for the printer can´t handle these several loose objects. It preferes one single, clean mesh without non manifold edges.

I already tried using the Boolean modifier to join the objects but it ended in a mess with lots of non manifold edges. 

Here a link to the .blend file i created.

Does someone have expierience with joining mulitiple objects into one single? Slowly I´m really annoyed from this problem and would be really pleased if someone could help me out with an solution for this.
Typing Ctrl+J should work. It is a shortcut that joins meshes. If there are any modifiers that are not applied yet, apply them then hit Ctrl+J.