After his sentencing, police said they were investigating whether Watkins, 36, committed further abuse in the US and Germany. The first allegations are understood to have been made against Watkins in 2008, four years before he was finally arrested.
IPCC commissioner Jan Williams said: "We are continuing to gather and analyse information in all three investigations in order to establish what steps were taken by police in response to the allegations made against Ian Watkins, whether he could have been brought to justice sooner and whether his celebrity status had any impact on those investigations."
The commission is investigating complaints around the handling of three reports made to South Yorkshire Police between March and May 2012 which contained allegations against Watkins with potential evidence.
A sergeant and two constables from this force have now been served with misconduct notices advising them that their conduct is subject to investigation.
The IPCC said it is also investigating Bedfordshire Police's handling of information from a member of the public who reported an allegation of child abuse against Watkins in October 2012. A sergeant and a constable have also been served with misconduct notices advising that their conduct is under investigation.
The South Wales Police detective constable, who has similarly been sent a misconduct notice, was attached to the Child Protection Unit, and is the second officer from the force to be investigated.
South Wales Police and South Yorkshire Police said they were cooperating with the IPCC.
Bedfordshire Police added: "Bedfordshire Police received a complaint from a member of the public after she had reported a suspected incident of child abuse.
"The force Professional Standards Department investigated the complaint and an email explaining the outcome of that investigation was sent to the complainant. At the time of the original investigation there was insufficient evidence to apply to the magistrates for a warrant.
"A medical examination of the child did not reveal any evidence of abuse at that time. However, information was shared with partner agencies, including South Wales Police and child protection processes followed."
- Independent