K3 started by Niels William. Initially, women in their twenties were the target audience, which is noticeable in early K3 song lyrics. Their first single, "Wat Ik Wil", was not very successful. In 1999, K3 participated with the song "Heyah Mama" in the live television shows to select the Belgian entry for the Eurovision Song Contest. The song was not selected but was released as a single and became the band's first big hit. About a year after becoming famous in Flanders, K3 also became famous in the Netherlands. Notable hits are "Alle Kleuren", "Hippie Shake", "Tele-Romeo", "Toveren", "Oya Lélé" and "MaMaSé!".
K3 is noted for being the first pop act to break through to the top of the charts without receiving any airplay from regular music television channels. Niels William's marketing concept was based on promotion via children and cartoon channels like Fox Kids. Since 2002 the band is managed by Studio 100, a Flemish television production company for Dutch and Belgian children's programs.
Kathleen Aerts left the group on 23rd March 2009.
On the 3rd Ocober 2009 K3 became Flemish/Dutch as the Dutch Josje Huisman was chosen as K3's new member through a reality show called K2 zoekt K3.
On 18 March 2015 at 3 PM, K3 officially announced that all three band members would stop and that the band would restart with three new band members selected through a reality show called K3 zoekt K3.
On Friday 6th November around 10.30 PM, after several selection weeks, the reality show winners, Hanne Verbruggen, Marthe De Pillecyn and Klaasje Meijer won the finals and form the 'new' K3. The blond member, Klaasje, is from Holland. Marthe and Hanne are from Belgium.
Two K3 groups will be simultaneously active from 6th November 2015 until 8th May 2016: the 'old' K3, Karen, Kristel and Josje appear on stage giving their farewell tour together with the 'new' K3 Klaasje, Marthe and Hanne.
From 9th May 2016 K3 continues with the renewed occupation Klaasje, Marthe and Hanne.