Maybe because in those three countries, only in Iran do the leaders say "Death to America"?

Cost and reward calculations where entered into and the cost of attacking North Korea was calculated as too high.. Same goes for the Indian calculation beck in the 1974...

In Irans case the reward was considered worth the possible cost..

Originally Iran was in a fragile pact with the USA, France, Germany and the UK that it would not develop nuclear weapons.*

Then President Trump unilaterally detroyed the USA's role in the treaty for no clear, but certainly no good, reason. With the Iranians thus betrayed by a country they do not particularly like anyway, at least ideologically, the other nations were unable to save the treaty.

The USA may imagine it can "stop" any country from doing anything - but it can not. If that country has the political determination and technical ability to make nuclear weapons, it will. Foreigners betraying an already very fragile truce then hurling high explosives at the country, only makes the situation worse.

The "nuclear arms race" was coined to describe the Cold War, in which the USSR and NATO competed to build ludicrous numbers of ever more powerful weapons for a war whose "victor" would be the one slightly less devastated than the other.

That never really ended; and I fear we are seeing a new one developing but with more countries involved, and in an increasing tangle of rival ideologies and ambitions.

+++++

* "Nukes". I refuse ever to use that awful word. Nuclear fission and fusion bombs are terrible weapons that should never be cheapened by slang on the level of that silly "Duck & Cover" cartoon tortoise.

Well one answer is that Iran is a signatory to the Non-Proliferation Treaty and therefore developing a nuclear weapon would be in breach of its treaty obligations. India has never signed the treaty and North Korea has withdrawn from it. There are, of course, other answers as well.

NK is too close to China to start dropping bombs. China would likely get involved.
India has way more people than USA. If they could figure out their country they could be battling for top spot along with China right now.

1) Clinton signed a "nuclear agreement" with North Korea when Little Rocket Man's daddy was dictator and didn't enforce it.

2)India isn't a rogue nation.

They did try decades ago

You're asking for logic, that won't do

I think trump knows that India and North Korea wouldn't put up with it.

Exactly it’s India they need to worry about not Iran

pakistan also have bomb