* Oil prices weaken for third straight day, Exxon down early
* Investors looking ahead to consumer confidence data
* Futures down: Dow 89 pts S&P 11.5 pts, Nasdaq 20 pts
By Ryan Vlastelica
NEW YORK, March 31 (Reuters) - U.S. stock index futures were lower on Tuesday, indicating a modest retreat from the previous session's rally, including the strongest session for the Dow in more than a month.
* Energy shares will continue to be in focus as crude oil fell 2.3 percent to $47.58 per barrel, sending Exxon Mobil Corp down 0.7 percent to $85 in premarket trading. Oil is set for its third straight daily decline, losing more than 7 percent in the three days. U.S. crude futures were volatile on Monday, swinging between a gain of 0.6 percent and a decline of 2.6 percent.
* Oil was pressured on Tuesday as Iran and six world powers entered a final day of talks on a nuclear deal that could see the energy-rich country increase oil exports to world markets. The talks come at a time when oversupply is already a major concern for energy investors, contributing to crude being down more than 50 percent from a high reached in June.
* Investors will also look ahead to the latest economic data, including the March Chicago Purchasing Managers Index at 9:45 a.m. EDT (1345 GMT) and a reading on March consumer confidence at 10 a.m. Confidence is seen dipping slightly in the latest month.
* Trading may be volatile ahead of the March payroll report, due on Friday, when the stock market is closed for the Good Friday holiday. If the report is strong, investors could bet the U.S. Federal Reserve may raise rates earlier than currently expected.
* In company news, International Business Machines Corp said it will invest $3 billion over the next four years in a new 'Internet of Things' unit, aiming to sell its expertise in gathering and making sense of the surge in real-time data.
* Office Depot Inc fell 1.1 percent to $9.25 in light premarket trading, a day after the company received a request for additional information from the Federal Trade Commission about its planned purchase by Staples Inc. Staples received the same request.
Futures snapshot at 6:58:
* S&P 500 e-minis were down 11.5 points, or 0.55 percent, with 118,464 contracts changing hands.
* Nasdaq 100 e-minis fell 20 points, or 0.46 percent, in volume of 17,777 contracts.
* Dow e-minis dropped 89 points, or 0.5 percent, with 20,424 contracts changing hands. (Editing by Jeffrey Benkoe)