Nord Stream is part of Russia's efforts to diversify export options for Europe. Spats between Kiev and Moscow make the conventional route through Ukraine politically risky.
The first leg of the dual pipeline is set for an official launch date of Nov. 8. The second leg of the pipeline is expected online next year.
Alexander Medvedev, deputy chief executive at Gazprom, was quoted by Russia's Itar-TASS news agency as saying a third line was possible.
"This is tentative discussion, we shall be prepared to supply to Europe as much gas as Europe may need," he said. "To see how much gas will be needed one has to estimate the demand. The ball is in the European court."
Each line of the Nord Stream project is predicted to carry about 970 billion cubic feet of natural gas per year. Europe gets about 25 percent of its natural gas from Russia.
Engineers working on Nord Stream connected the first leg of the twin pipeline to Russian and German networks in August. Construction through Baltic Sea began in early 2011.