The 3I is a loose, vulnerable sprawl of worlds, which by the way is no worse off than the other interstellar states. Interplanetary trade exists on major worlds. Starports are significant on major worlds but not so massive. The 3I is a picture of gross mismanagement over impossible distances. It's a wonder it held together so long.
Interplanetary trade exists on only the best worlds.
3i IMTU, the 3I doesn't exist or is irrelevant. Well, we have jump drives, but I never had to explain them. FTL drive mechanics are a bit different out here. We ride the sky in real slow beasties. Jump torpedoes are not only canonical, but absolutely necessary for survival as we know it.