here are a few witness accounts on the 3rd shooter, there are about a hundred accounts but text limit.
According to the final report of the investigation, only two people–Eric Harris, 18, and Dylan Klebold, 17–planned and executed the mass shooting at Columbine High School outside Littleton, Colorado, on April 20, 1999, leaving 15 dead and 23 wounded. But at least 101 eye- and ear- witnesses remembered it differently:
1) Chris Wisher, sophomore (1261) He turned and saw two guys carrying shotguns, and wearing black trench coats. A 3rd guy wearing a white T-shirt and jeans was seen throwing bombs onto the roof.
2) Jake Apodaca, sophomore (653) He was at the soccer field, heard firecrackers. He turned and saw two guys carrying shotguns, wearing black trench coats and black T-shirts. A 3rd guy wearing a white T-shirt and jeans was seen throwing bombs onto the roof.
3) Jonathan Cole, freshman (749) He noticed two tall guys in black trench coats. He then saw a third male, wearing a white T-shirt and blue jeans. This person looked over to the other two suspect, and yelled "GO!! GO!!".
4) William Arapkiles, freshman (660) He states that he was at the soccer field, and heard firecrackers. He turned and saw 3 people near the West entrance. Two were wearing black trench coats, and carrying guns. The third was wearing a white T-shirt, and no visible gun. The person in the white T-shirt was pacing back and forth about 5' away from the two gunmen.
5) Channel 7 News at 12pm(noon) on 4-20 (EP 21-244) "Reporter said that numerous witnesses he spoke to who were not named said they saw 3 gunmen."
6) Lindsay Elmore, junior (332)"I believe there were 3 gunmen." (340) "…there was so much movement that Elmore thought there was more than two people."
7) ACSO Nelson (8810) "… a list of 3 names that the 12 witnesses ID'd from a yearbook were turned over to ACSO investigators to forward."
8) Jason Baer, sophomore (1759) Hid in greenroom off science hall "I heard more than two people yelling to each other saying 'over here' and stuff."
9) Anthony Sammauro, freshman (5071) "Tony stated that two suspects came in the front door and one came in the side door."
10) Erik Sunde, freshman (4576) Saw two gunmen downstairs near cafeteria, runs upstairs and sees another person in trenchcoat in front of library doors.
11) Jon Curtis, staff (2286) Describes 3 attackers.
12) Richard Olejniczak, father of students Alicia and Sarah (5070) "Richard then asked this Investigator what information had been obtained reference any other participants. Richard stated that while he was at Lea Wood Elementary School, waiting for information about his two daughters, he talked with two girls who came from the school. The girls said they had seen two guys in the cafeteria, wearing all black. There were also two guys who entered the main door from the parking lot. One was in black, the other in white. The girls said they all had guns.
13) Adam Campbell, senior (732) Walked out of school at 11:20am(immediately before shooting) on 4-20 and saw Harris in the parking lot with 2 or 3 other guys, wearing a white T-shirt and a backward baseball cap.
14) Matt Katzenmeier, sophomore (5745) Saw 4 subjects carrying bags in, according to Darlene Mesch.
15) Alicia Maes, sophomore (22755) Saw 4 people with trenchcoats and guns, according to Rachel Nelson.
16) Natalie Baker, freshman (2583) Told her mother there were 2 other shooters besides Harris and Klebold.
17) Jennifer Tindall, sophomore (1226) Sees 2 suspects, neither of which is Harris and Klebold.
18) Ryan Ezzie, sophomore (2967) Saw 4 people outside with hats on backwards. On 20612, he says he "…saw at least 4 parties dressed in black standing calmly when others were running."(by the school library)
19) Jordan Grimm, freshman (22432) According to Jordan's aunt, he saw Harris and Klebold and 2 others together ouside school just before the incident began.
20) John Spahlinger (1176) Clement Park maintenance worker, "a majority of the students Spahlinger helped informed him there had been a trench coat mafia gang shooting over at the high school and that there were 4 shooters involved."
21) Thornton/NorthglennPD SWAT Team (EP 9-34) At 11:35pm on 4-20(12 hours later) the team enters CHS for another search. "We were advised that 2 suspects(shooters) might still be in the school."
22) Littleton PO Mike Eyman (8670) Advised that "there were possibly 4 suspects in the school."