* Full confirmation expected to occur by Memorial Day recess - sources

* Two board seats remain vacant

* FOMC in Day 1 of a two day meeting; next meeting is June 17-18 (Adds Breakingviews link)

By Michael Flaherty and Richard Cowan

WASHINGTON, April 29 (Reuters) - The Senate Banking Committee approved three nominees to the Federal Reserve's board on Tuesday, including Stanley Fischer to be the U.S. central bank's No. 2, in a big step toward replenishing the Fed's governing body.

The panel also backed the nominations of former senior U.S. Treasury official Lael Brainard and current Fed Governor Jerome Powell, who was nominated for another term. All three nominees were approved on a unanimous voice vote.

The nominations are now cleared to go before the full Senate for final confirmation votes. A date for Senate consideration has not yet been officially set.

Former Fed officials say the Senate needs to act fast to confirm the nominees, given the heavy workload the board faces with only a few governors in place.

People familiar with the matter said that a full confirmation of the three is expected by the Memorial Day recess in late May, though it was still unclear whether Democrats and Republicans will agree on the timing.

With Fed Governor Jeremy Stein leaving in late May to return to academia, the normally seven-person board would fall to three members for the first time in its more than 100-year history if the Senate does not move quickly.

"If the Fed goes down to three in June, it's reprehensible," said Northern Trust economist Carl Tannenbaum, who has worked for both the Fed board in Washington and at the Federal Reserve Bank of Chicago.

Fed policy makers began a two-day meeting on Tuesday. At its previous meeting, the board voted to reduce the monthly pace of bond purchases by $10 billion, to $55 billion, and said more reductions in "measured steps" were likely. The next FOMC meeting is set for June 17-18, which includes Yellen's second press conference.

The Fed also has 12 regional branches, whose presidents rotate through five voting positions each year.

Fischer's previous roles include the No. 2 spot at the International Monetary Fund, chief economist at the World Bank and vice chairman at Citigroup.

The 70-year-old, who was born in Zambia, is highly regarded in monetary policy circles for his extensive international experience, his academic career teaching economics at the Massachusetts Institute of Technology, and his successful stint at the top of Israel's central bank.

The combination of veteran Fed policymaker Janet Yellen leading the central bank and Fischer as second in command comes with high expectations, though Fed watchers will have to wait and see just how well the two work together in their roles.