"We would have it to talk about nuclear weapons and denuclearisation," Trump told reporters outside the White House on Monday.
"If President Xi wants to have it, I'd have it. I'm sure we'd talk about other things like trade too."
Trump has tried to thaw frosty relations with Beijing and Moscow as those non-Western powers grow closer. A three-way meeting would also raise alarm from Europe to Asia over private deals that weaken Washington's transatlantic and Indo-Pacific alliances.
But a deal struck between the three countries could also lower global tensions.
An end to Russia's war in Ukraine could resolve the biggest threat in decades to European security, and a deal on arms would bind the three nations with the biggest stockpiles of nuclear weapons.
In recent days, Trump has moved to warm ties with Russian President Vladimir Putin, announcing a diesel-fuel deal and signing an order on Friday waiving key sanctions on Moscow.
Trump has also sought stable relations with Chinese President Xi Jinping, who in turn has nurtured close ties with Putin.
So far, Trump has not delivered on a campaign promise to quickly end the war in Ukraine.
The White House has been discussing for weeks a possible meeting between the leaders focused on denuclearisation on the sidelines of the November 18-19 Asia-Pacific Economic Co-operation summit in China, according to a person familiar with the matter.
Putin later pitched using such a meeting to negotiate an end to the four-year-old Ukraine conflict in a manner that would favour Moscow, the Wall Street Journal reported.
Putin's suggestion came during an October 9 phone call with Trump that yielded the deal on Russian diesel.
Earlier on Monday, the Kremlin said that the potential meeting had come up in talks.
The three leaders have met separately but never held formal talks together.
Speaking about the possible meeting on Monday, Trump said, "I would do it, not so much for Ukraine because Russia, frankly, has been fine on Ukraine".
"(Ukraine President Volodymyr) Zelenskiy is not helping matters much," he said.
"It's time for somebody else to negotiate on behalf of Ukraine."
Moscow launched one of its broadest air assaults in months overnight, cutting power to more than half a million households and damaging energy infrastructure across Ukraine. Ten people were reported killed.
