NEW YORK (Reuters) - Mexican tycoon Carlos Slim retained his position atop Forbes magazine's annual list of the world's billionaires on Wednesday with an estimated worth of $69 billion, placing him ahead of Bill Gates at $61 billion and Warren Buffett at $44 billion. Slim, 72, has taken the top spot for three years in a row, largely based on assets from his telecommunications empire. The United States again led the list with 425 billionaires. Russia overtook mainland China for the No. 2 spot, with 96 billionaires compared with mainland China's 95. Moscow led all cities with 78 billionaires, followed by New York at 58 and London at 39. This year's list, in the magazine's 25th annual issue on the world's wealthiest people, included a record 1,226 entrants who sat atop average fortunes of $3.7 billion. Slim made much of his money as a telecommunications magnate who has expanded into retail, finance, commodities and energy. In recent years he has taken larger stakes abroad, owning parts of department store operator Saks Inc, publisher New York Times Co and money manager BlackRock Inc . Gates narrowed the gap with Slim this year after Slim's estimated net worth fell from $74 billion to $69 billion. Forbes estimated Gates' net worth rose from $56 billion to $61 billion. Reporting By Daniel Trotta and Edith Honan; Editing by Paul Thomasch and Matthew Lewis