Harbaugh was among the most sought after coaches this winter following a 12-1 season in his fourth year at Stanford capped by a 40-12 Orange Bowl victory over Virginia Tech.
"I am very excited that these three men are now part of the 49ers coaching staff," Harbaugh said in a statement. "They bring a wealth of knowledge and a level of professionalism that I am certain will transfer positively to our team. Vic has 24 NFL seasons under his belt and Greg has 13, so they know firsthand what it takes to win at this level."
San Francisco (6-10) missed the playoffs for the eighth straight year this season after the Niners were the popular pick to win the NFC West. Instead, they began 0-5 and couldn't recover -- though they were still in playoff contention in the weak West until a 25-17 loss at St. Louis on Dec. 26. Singletary was fired later that night with two years remaining on his contract.
New Stanford coach David Shaw acknowledged during his introductory news conference Thursday he would have key spots to fill on his staff.
Roman replaces Mike Johnson, who took over as offensive coordinator after Jimmy Raye was fired following a Week 3 loss at Kansas City. Roman will be the Niners' ninth different offensive coordinator in as many years including Johnson's promotion for most of the 2010 season.
Roman guided a Cardinal offense that ranked ninth nationally in points scored this season at 40.3, 14th in total yards per game (472.5), 17th in yards rushing per game (213.8) and first in time of possession (34:34).
He and Harbaugh will likely have a new quarterback to groom right away. Alex Smith, the 2005 No. 1 overall draft pick out of Utah, becomes a free agent.
Fangio, 52, takes over for Greg Manusky, who was hired as defensive coordinator of the San Diego Chargers on Friday. This will be Fangio's 25th season as an NFL coach -- and he has four-time Pro Bowl linebacker Patrick Willis as the anchor of his experienced and talented defense. Fangio spent 11 years as a defensive coordinator with Carolina, Indianapolis and Houston.
Drevno spent all four seasons on Harbaugh's Stanford staff, and his line allowed only 13 sacks the past two years.
Stanford returned to a bowl game in 2009 for the first time since '01 behind Gerhart and Luck. The Cardinal beat Oregon and USC in consecutive weeks to move into the AP poll for the first time since 2001, stayed in contention for a Pac-10 title until late in the season. Stanford lost the Sun Bowl to Oklahoma when Luck sat out with a broken right index finger.
Now, the 49ers coaches will try to duplicate that Stanford turnaround at the next level. San Francisco hasn't had a winning season since its last trip to the playoffs in 2002.
The Associated Press contributed to this report.