Let's break it down for these characters, based on Marvel statistics of March 11, 2019.
- Speed: 7,620 miles/hour
- Energy: 418,000,000,000,000+ joule
- Durability: 1,000,000,000,000,000 joules
- Strength: 200,000 pounds
- Speed: 240,000 miles/hour
- Energy: 418,000,000,000,000 joules
- Durability: 4,180,000,000,000,000 joules
- Strength: 400,000 pounds
- Speed: 640,600,000 miles/hour
- Energy: 2,000,000,000,000,000 joules
- Durability: 3,86 * 10^26 joules
- Strength: 2,000,000,000,000,000 pounds
As you can see:
- Iron Man is just an earth man and relies on his suit. Which means he is slower, can't lift as much and can't take a punch like the other 2, but his energy output is on par with Captain Marvel.
- Captain Marvel is an augmented earth woman. Her Kree powers give her more durability and a higher speed and strength than Iron Man. But they both are somewhat equal in energy output.
- Thor is Norse GOD. He clearly outclasses the others in terms of speed (he can teleport), strength (lift a trillion ton) and durability (withstand the nonillion joule output of the heart of a star). His energy output is closer to the others in terms of absolute value, but it is still about 4-5 times higher than both Iron Man and Captain Marvel.
--- EDIT ---
The article linked is creating confusion for some.
As the title states that "Captain Marvel is the most 'powerful' hero".
Unfortunately this is only to create a buzz. (The article was written for the Captain Marvel movie release and thus needed to give a boost to the character)
Because if you read the article and look at the data presented, you get a different conclusion.
A more correct title would have been "Captain Marvel is the best overall hero", but that doesn't have the same ring to it.
Why the best overall character? Because Captain Marvel is not the best in any field (Hulk is stronger, Thor is more durable, Iron Man is more intelligent, Captain America is a better strategist, ...), but she is always in the higher ranks in every field and thus overall beter.
Based on the article, I created the ranking per field for some of the most powerful heroes and added the winner per field for the 3 heroes of this question (Iron Man, Thor and Captain Marvel).
The question was about the most powerful energy blast and we can see that Thor is the winner for that category.
--- Edit 2 ---
"Energy output for Cap being zero seems weird. compared to others, Hulk doesn't have any energy beam or anything, just like Cap. but him having a huge number and Cap having zero is unclear to me why."
Nor the Hulk nor Captain America have any energy beams or rays or anything like that.
But the difference is that the Hulk ... euhm ... Hulks out. Bruce Banner transforms into the Hulk and gets stronger and stronger as he gets angrier.
He goes from a weak man with a mass of 128 pounds (58 kilograms) to an indestructible behemoth weighing in at 1150 pounds (521 kilograms).
This change in mass can only be achieved by conversion of energy to mass through massive amounts of energy. This explains his high number (although realistically the number would have to be even higher to explain it via E= mc2).
"In Speed, Hulk can jump a long distances, since this answer based on comics, you should consider that and I think that makes his speed more than 473."
This answer is based on a Marvel article using their stated number for the movie version. Because in the comics the Hulk has moved at the Speed of Thought (a speed of 31.56*1027 times the speed of light), but that is just ridiculous and is based on what the writers need at that moment. The movie Hulk has a ground mobility of over 400 mph. Which I still find absolutely impressive. A 1150 pounds heavy angry Hulk running at you at over 400 mph? You wouldn't like that. Maybe you want him to be faster, just so it would be over faster.