This provoked my first post here...
I'm in the middle of my own build (quite similar to yours) and I'm using 100x100x10 so I'd throw in a few words... Don't overlook the edge radius of your SHS. I got unpleasantly surprised when I was supplied with 100x100 SHS that has 35mm (or perhaps even more) diameter at edges... I was counting with 25mm diameter I found in tables of SHS somewhere but the supplied steel was cold rolled with much larger edge radius. My rails will be right at the edge of square so I added thick plate on top of SHS to support the rails. You may not have this problem with 3mm wall but I'd check at the supplier and verify the design with that in mind.
Good luck!
Adrian