WASHINGTON — President Obama will soon announce that the United States will contribute $3 billion to a new international fund intended to help the world’s poorest countries address the effects of climate change, according to a senior administration official. Mr. Obama is expected to make the announcement at a summit meeting of the Group of 20 industrial powers this weekend in Brisbane, Australia, on the heels of his landmark announcement this week that the United States and China will jointly commit to curbing greenhouse gas emissions over the next decade.