This matters because those three are on the Senate Banking Committee. If they vote "no," Summers needs a Republican vote to get out of committee, and that's not likely. This could kill any Summers nomination.
Greg Sargent provides a report from the Wall Street Journal (the WSJ article is behind a pay wall -- the link is in the Sargent article -- you can follow my link)
The Wall Street Journal scoops that three key Dems on the Senate Banking Committee will vote No if Obama nominates Larry Summers for Fed chair: Sherrod Brown, Elizabeth Warren, and Jeff Merkley. A source familiar with Brown’s thinking confirms to me he’s a No.
Since Dems only hold a two-vote majority on the committee, that would mean Obama would need GOP support to get him to the full Senate, which, given recent history, seems unlikely — even though Fed chair picks are traditionally given wide bipartisan support.
WaPo, the Plum Line
A good reason not to nominate Summers. President Obama does not need that fight. It is better to unify Democrats with a different nominee.