House Minority Leader Nancy Pelosi added Rep. Xavier Becerra from Los Angeles, along with James Clyburn of South Carolina and Chris Van Hollen from Maryland.
"It is an honor and a great responsibility to be entrusted with this assignment by our Leader Nancy Pelosi," Becerra said in a statement.
The choices bring racial diversity to the supercommittee because Clyburn is black and Becerra is Hispanic.
The appointments complete the 12-member panel's roster, made up evenly of Democrats and Republicans.
On Wednesday, House Speaker John Boehner, R-Ohio, named conservative Rep. Jeb Hensarling, R-Texas, a rising force among House Republicans, as GOP co-chairman of the powerful new panel. He also appointed House Ways and Means Chairman Dave Camp and House Energy and Commerce Chairman Fred Upton, a pair of veteran Michigan Republicans, to the committee.
Senate Minority Leader Mitch McConnell, R-Ky., named confidant and No. 2 Senate GOP leader Jon Kyl of Arizona, tapping a lawmaker who is retiring in 2013 and is a solid conservative. He also appointed GOP Sen. Pat Toomey of Pennsylvania, elected last year with tea party backing, and fellow freshman Sen. Rob Portman of Ohio, a former budget director and trade representative for President George W. Bush who is viewed as a possible vice presidential pick next year.
On Tuesday, Senate Majority Leader Harry Reid, D-Nev., chose Sen. Patty Murray, D-Wash., who runs the Senate Democratic campaign arm, as Democratic co-chair of the debt committee. He also appointed 2004 Democratic presidential nominee and Senate Foreign Relations Committee Chairman John Kerry, D-Mass., and Senate Finance Committee Chairman Max Baucus, D-Mont., a centrist who strayed to back Bush's 2001 tax cuts.
The panel has until Thanksgiving to come up with a plan to cut $1.5 trillion from the U.S. budget, or automatic budget cuts will be triggered.
The Associated Press contributed to this report.