Not exactly, although they contributed a lot to the early work. If we start from "splitting the [heavy] atom", the first practical method, a German radiochemist was the first to recognize what happened, and the German physicist he first forwarded this information and her nephew figured out how it worked. But those two were Jews in exile, and most of the following more practical stuff was figured out, correctly at least, by scientists outside of Germany. Inside, they suffered a couple of disasters, a silly failure to get an higher priority for the project, and an incorrect evaluation of using graphite as a moderator, limiting them to heavy water. Which we made sure through sabotage that they couldn't get more/enough of.

But if you put on your alternate history cap, note Heisenberg and Himmer's relationship from when they were school kids, direct all the V-2 resources into the bomb, who knows? Many on both sides were legitimately worried the Germans would get the bomb in time.

As for the Japanese, no way. Besides what you read in the article, thermal diffusion was all but a failure in the US, among other things required fantastically too much steam, and was only revived to provide somewhat enriched feed stock into the caltrons (modified cyclotrons), and later into the gaseous diffusion plant before it fed the caltrons. ADDED (forgotten while drafting): centrifuges were of course obvious, and about the first thing we tried, but were beyond the state of the art then.

1,200 pounds of uranium at the normal 0.72% U-235 would not come close to yielding one critical mass, and for gun assembly apparently you need more like 3 times the normal pressure critical mass, although it doesn't all have to be of the highest grade.