But 3 nukes in a triangle near the ocean into the crust like on Maldek could eventually destroy the whole planet

How?

Because a planet like Earth is mostly water on top of the crust. If you look at a map of earth you are going to see water. And there is more water than there is land above the water. Carefully look at a map of earth.

So, what would happen if three nukes blasted deep enough on the land into the crust near the ocean or even under the ocean is that if the hole in the crust that resulted was big enough it could start through steam explosions a chain reaction of these explosions that eventually unbalanced the planet. when the planet got unbalanced enough it would start to throw off atmosphere, water moisture and pieces of dirt and rocks out into space from the unbalance.

This actually happened to Maldek the planet in a nuclear war 65 million years ago and a piece ejected into space came all the way to earth and killed all the larger dinosaurs then too. Just look at the asteroid belt on a map of the Solar System and you can see the result now 65 million years later of what actually became of the planet Maldek from a nuclear war by our ancestors then. 

However, it took several months for billions of people to die in various ways until the planet was gone and then it became the asteroid belt out past Mars.