Blender question - bottoms

So I am trying to edit a Maxis bottom and I’m running into a blender issue that may or may not be resolvable. I want to edit all three sections at once but when all three are selected in object mode, the last one selected only being editable in edit mode. Usually in blender all three would be editable in edit mode when selected.

Thanks in advance!

How do I work on all three in edit mode?