Bezos will paint online giant Amazon as an American "success" story, while accepting a need for scrutiny.
"I believe Amazon should be scrutinised," Bezos said in prepared remarks posted online ahead of the hearing.
But he struck a defiant note, too, saying that "when you look in the mirror, assess the criticism, and still believe you're doing the right thing, no force in the world should be able to move you."
It will be the first time Bezos has testified before Congress.
Zuckerberg acknowledged "concerns about the size and perceived power that tech companies have."
"That's why I've called for a more active role for governments and regulators and updated rules for the internet."
Pichai will highlight how Alphabet is helping America solidify its position as the global leader of budding technologies such artificial intelligence and self-driving cars.
"Just as American leadership in these areas is not inevitable, we know Google's continued success is not guaranteed," Pichai said in prepared remarks.
"Google operates in highly competitive and dynamic global markets."
Pichai will also point out that people have many options other than Google for online searches.
Cook will say scrutiny is "reasonable and appropriate" but that Apple will make "no concession on the facts."
Apple does not have a dominant position in any market, and the iPhone faces fierce competition, according to Cook's prepared remarks.
"Apple is a uniquely American company whose success is only possible in this country," Cook said.
Big vs Bad
The showdown in the House of Representatives comes amid rising concerns over Big Tech dominance, which has become even more pronounced during the coronavirus pandemic.
"Ultimately, I believe companies shouldn't be making so many judgments about important issues like harmful content, privacy, and election integrity on their own," Zuckerberg's prepared remarks read.
Questions at the hearing are expected to veer into issues ranging beyond whether the technology titans are abusing their power in marketplaces.
Current US antitrust laws make it difficult for enforcers to target companies simply for being large or dominant without also showing they cause harm to consumers or abuse their market power.
"We believe in values -- democracy, competition, inclusion, and free expression -- that the American economy was built on," Zuckerberg is to say.
"Many other tech companies share these values, but there's no guarantee our values will win out."
He is to cite the example of China building and exporting a version of the internet focused on "very different ideas" from the US model.
"I believe it's important to maintain the core values of openness and fairness that have made America's digital economy a force for empowerment and opportunity here and around the world," Zuckerberg is to say.
The tech company chiefs are expected to stress how they benefit consumers, particularly during the pandemic, and face competition -- particularly from China.
Bezos will also spotlight increasing competition among online retail platforms.
"Our success may help explain the wide proliferation of marketplaces of all types and sizes around the world," Bezos said.
"This includes US companies like Walmart, eBay, Etsy, and Target, as well as retailers based overseas but selling globally, such as Alibaba and Rakuten."
The antitrust debate is being muddled by a rising "techlash" over a range of issues from privacy to economic inequality to political bias.
"As I understand our laws, companies aren't bad just because they are big," Zuckerberg is to say.
"Many large companies that fail to compete cease to exist."
Social media giants face attacks for allegedly using their dominance to stifle conservative views -- a claim made by President Donald Trump.
Facebook has also been accused of failing to curb hateful content promoting violence, including from Trump.