* Oil prices weaken for third straight day, Exxon down early
* Charter to buy Bright Hose for about $10 bln: source
* Investors looking ahead to consumer confidence data
* Futures down: Dow 88 pts, S&P 10.5 pts, Nasdaq 20 pts (Updates prices, adds M&A deals)
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, which was 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, having contributed to crude falling 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 view the U.S. Federal Reserve as more likely to raise rates earlier than currently expected.
In company news, Endurance Specialty Holdings Ltd agreed to buy reinsurer Montpelier Re Holdings Ltd for about $1.83 billion, while a source told Reuters that Charter Communications Inc has agreed to acquire Bright House Networks in a roughly $10 billion deal.
The news follows a number of big M&A deals announced on Monday that contributed to the market's broad advance. Charter rose 11 percent to $203.50 before the bell on Tuesday.
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 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 8:18:
* S&P 500 e-minis were down 10.5 points, or 0.51 percent, with 160,096 contracts changing hands.
* Nasdaq 100 e-minis were down 20 points, or 0.46 percent, in volume of 24,035 contracts.
* Dow e-minis were down 87 points, or 0.49 percent, with 27,608 contracts changing hands. (Editing by Jeffrey Benkoe and Meredith Mazzilli)