3 Idiots

 Anime= Manga= Three bullies from Osamu and Yūma's school, who are collectively known as 3 Idiots (３バカ) aka the Idiot Trio. In the manga, their names are not given and are referred to as No.1, No.2, and No.3. In the anime, the credits gave them the surnames: Eida, Bitō, and Shīhara.

they all appeared in the first chapter and first episode

 Eida=

Bitō=

Shīhara=

Trivia

 * No. 1 (Eida the redhead) is "Ringleader" of the group. He is the worst of all the delinquents but Ashihara wishes to improve his danger detection skills so he could survive.
 * His Japanese voice actor, Mitsuo Iwata, also voices Isami Tōma.
 * His English voice actor, Bill Newton, also voices Isami Tōma, and Kōtarō Suwa.
 * No. 2 (Bitō, brown hair) is "The Annoying One", The author doesn't exactly hate him however he finds him easy to draw.
 * His Japanese voice actor, Hiroaki Miura, also voices Ryō Utagawa.
 * No. 3 (Shīhara, dyed hair) is "Roots Showing". According to Volume 1, his delinquency is his hobby. He has good grades, and his family is quite rich.
 * His Japanese voice actor, Kōta Nemoto, also voices Ken Satori, while his English voice actor, Daegan Manns, also voices Reiji Kizaki.