Anheuser-Busch InBev NV cut its revenue forecast for the year after the world’s largest brewer reported weak third-quarter results, dragged down by surging financing costs and slumping sales in Brazil and the U.S.

The worse-than-expected performance ratchets up investor scrutiny of AB InBev as it races to digest its $100-billion-plus takeover earlier this month of chief rival SABMiller PLC—a massive bet that it can tap new growth in Africa and other emerging markets.

The...