Starports are planetary trade centers. The 3I is a stable confederation, held together by the nobility of its population, but not all-powerful and overall not wealthy. Interplanetary trade is regular. Starports are common. 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.