I suppose it depends on how exactly one views the concept of Fallout.
Fallout 1 is the same engine but a very different feeling game from Fallout 2.
Well, yes and no. I think Fallout 2 suffered from a lack of direction, that lack of direction resulted in the ridiculous humor (and the ridiculous amount of said humor) present in the final product. Fallout 2 still felt like Fallout to me though; it was basically Fallout 1 with more content (some better, some worse). It adhered to the general design philosophy of the previous game even if the setting suffered a bit. It's by no means a perfect sequel, but I think it had less issues than Fallout 3 did in regards to its status a Fallout game.
Both are very different from their spiritual father of Wasteland.
I don't think that really matters. I've never really considered Fallout a spiritual successor to Wasteland because the only real similarity between the two franchises is that they're post apocalyptic cRPGs with some of the same guys on board.
Fallout: Tactics, I point out is an entirely different game as well.
Fallout: Tactics is irrelevant I feel, it's a tactical spin off of the franchise and not a cRPG that heralds itself as a sequel. The setting was poorly handled, but I'm somewhat indifferent towards Tactics as I didn't really like it on its own merits, so I don't really care either way.
Fallout 3 is well... what it is.
Privately, I think it's a very different sort of game (being focused on combat) than Fallout 1 and 2. However, it's very much set in the Fallout world. What areas did you think it failed to deliver?
My biggest gripe was the lack of quests that branch off and allow you to make use of different skills in order to complete them (instead Fallout 3 focused more on a direct combat approach for nearly all quests with the occasional speech check at the very end). There was Tenpenny Tower, but that's the only one I can think of.
As for the world, I don't know... it felt too much like an idealized fantasy to me with all of its 'cool stuff' as Todd Howard put it. Fallout has always had 'cool stuff' yes, but generally this cool stuff was mixed into a world that had some degree of grittiness and realism to it. Fallout 3 didn't feel particularly gritty or believable because of all of the asinine places like Little Lamplight, and all of those epic quests you undertook weren't combined with more down to earth side quests. I don't think of Fallout 3 as non-canon or anything, but I do have difficulty at times accepting that it takes place in the same world as Fallout 1/2 because it feels more like a cartoon or caricature than a Fallout game. Although to be fair, I also tend to have difficulty believing that Fallout 2 takes place in the same world as Fallout 1.