Agnes Gund, an heiress who died last September, at 87, was a titan of generosity. In fact, The New York Times once wondered if Gund was the last good rich person. She gave away more than 1,800 works to museums, and used the $165 million fetched by her most prized Lichtenstein, Masterpiece, to fund criminal justice reform. By the end, she said, almost nothing was left. Still, there were three works that hung in her Upper East Side apartment: a Mark Rothko, a Cy Twombly, and a Joseph Cornell. They are now coming to Christie’s on May 18. The headline lot is Rothko’s No. 15 (Two Greens and Red Stripe), a monumental 1964 canvas in forest green and indigo anchored by a red-orange stripe. Gund bought the painting directly from the artist in 1967. Estimated to sell for $80 million, it has left her apartment only once, in 1972, for a one-month loan to the Cleveland Museum of Art. The Twombly is estimated to go for $40 to $60 million, and the estimate for the Cornell, a 1948 assemblage from his “Medici” series, is $3 to $5 million. —Elena Clavarino