The richest Americans collectively gave a total of $3.5 billion in 2023 to universities, scientific research institutes, a health care system, a family foundation and a racial justice group, according to the Chronicle of Philanthropy’s annual list of the year’s biggest charitable donations. Warren Buffet, the investment guru with an estimated net worth of $119 billion, topped the list by donating about $541.5 million in Berkshire Hathaway stock to the Susan Thompson Buffett Foundation, in honor of his first wife. His gift heads a list of the largest donations from individuals or their foundati…