It is beyond the scope of these posts to fully discuss the differences between Traveller Books 1-3 and the later editions of the core rules (The Traveller Book and Starter Traveller.)
That said, because The Traveller Book is easy to find in print, and both The Traveller Book and Starter Traveller have a better presentation of tables than found in Books 1-3, these later editions of the game are often the “go-to” editions for current Traveller players.
Because of this I want to touch on some of the concrete differences between Books 1-3 and the later editions that I think produce the biggest conceptual or philosophical differences. The rules may be the same, but the impact of information and text makes reading them very different experiences. Again, this is not a survey of differences, simply a look at broad stroke differences.