Who are they, and do they have anything to do with season 3?

They are P4, from Weston Academy. No, they don't have anything to do with season 3. They're from the mangá too, but is another part.

oh im not sure if they have something ro do with season 3 but... they are from this link
and the page that they get introduced is link
and they r the "Prefects"

hope i helped

They are the prefect 4 from the mangá at Weston college. They are (from left to right) Gregory tolet, violet from the tolet, violet lobo house, Edgar Redmond from the scarlet raposa house, Herman Greenhill from the green lion house, and Lawrence Bluer from the sapphire owl dorm. You can find the mangá at www.mangahere.com

To be honest, season 3 is actually Season 1.5.

Season 3 takes place after episode 15 of season 1.

Some say that season 3 retcons episodes 16-24 of season 1, all of season 2, & all of the OAVs.

Others say that season 3 is "lost episodes" that take place BETWEEN episodes 15 & 16.