sci-fi jeep

I can not "Boolean - union" 3 parts that make up a very simple Sci-Fi car body. I will be wanting to bevel selected edges where they join after it is union'd. The list of possible causes from the internet is beyond me at this stage past checking normals and double verts ?. This is probably very simple for someone who knows the problem. If there is a fix I would like to know what that problem was please.