How to download an stl file composed of several parts and not just the first one ?

Hi, I would like to download the attached file as an stl in high resolution which is composed of 2 small pieces and a third bigger piece, but when I do so in the website workbench it only convert the first small part in the stl.

Could someone tell me how to delete or join the two small parts to the big one ? Or if it is impossible to do in the workbench could you do it for me as I would like to not download the software as I don't need it for anything else.

Thanks a lot in advance