April 20 (Reuters) - Afterpay said on Tuesday it was exploring a U.S. listing without disclosing a timeline, as the Australian buy-now-pay-later firm’s third-quarter sales volumes more than doubled and beat market expectations. The Melbourne-based fintech reported underlying sales, the value of transactions it processed, of A$5.2 billion ($4.04 billion) in the three months ended March, 104% higher than a year earlier. ($1 = 1.2885 Australian dollars) (Reporting by Nikhil Kurian Nainan in Bengaluru; Editing by Arun Koyyur)