Personally, I think Fallout 3 was actually fairly faithful in its transition to 3D Action RPG.

Most of the mechanics present derived from the originals such as how Skills and SPECIAL were handled. It made sure to call back to how combat acted in the original with the VATs system and making AP a core resource. It actually did its research on the lore and characters of the series, such as when they wanted the BoS to be a more hero like faction they did a great amount of justification by having the Outcasts exist and making the BoS still be very condescending about locals and wanting to secure technology. And it tied back to the original by making the main conflict about water, and even gave thought to the Super Mutants by giving them a goal because their supply of FEV is limited essentially leading them to the end you convinced the Master of, where as a race they are doomed to die out cause they can not effectively reproduce.
True there were some things that were not so well thought out, I don't remember any explanation of how Little Lamplight maintains its population (I would have to guess kidnapping...?) and the Enclave almost felt like they were entirely disconnected from the events of Fallout 2 when it would have been nice to feel it be more connected with more hard references to them having regrouped in the Capital as their last ditch effort or something. But overall, I felt Fallout 3 was a faithful adaptation in line with the series, unlike 4 which needed like a few more months to a year of development for its writing, especially since somehow Elder Maxson of all people just forgot the Codex existed and you have no opportunity to actually levy the Codex and there was none of the finagling of factions you could do in NV or 2, as well as a certain detachment from the series themes as a whole.