Seven-year-old Elsie Dot Stancombe was killed along with Bebe King, six, and nine-year-old Alice da Silva Aguiar in an attack in Southport by Axel Rudakubana.
The public inquiry will look at whether the attack could or should have been prevented considering what was known about the killer.
Following the incident, it emerged Rudakubana, who was born in Cardiff, had been referred to the government’s anti-extremism Prevent scheme three times before the murders.
A rapid review into his contact with Prevent found his case should have been kept open and that he should have been referred to Channel, another anti-terror scheme.
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Rudakubana,18, was jailed for a minimum of 52 years in January for the murders of the three young girls and attempted murder of eight other children and two adults.
Chairman Sir Adrian Fulford, a former vice-president of the Court of Appeal, will begin proceedings with an opening statement at Liverpool Town Hall.
Sir Adrian said: “Today, just less than a year since one of the most horrific crimes in our country’s history took place in Southport, we open the independent inquiry into the events surrounding the attack and events leading up to it.
“Tomorrow, I will then begin to hear from some of the families whose children were injured on that terrible day.”
Two days of hearings will be held this week, and the inquiry will then resume on 8 September at Liverpool Town Hall.
The inquiry will then move on to a second phase next year to “consider the wider issues of children and young people being drawn into extreme violence”.
Sir Adrian added: “As chair of this inquiry, I am committed to proceeding at pace and with rigour whilst balancing the needs of those who live with the continued trauma of what happened in Southport in July 2024.”
Rachael Wong, director at law firm Bond Turner, representing the three bereaved families, said: “We know that nothing the inquiry reveals or subsequently recommends will change the unimaginable loss felt by the families of Elsie, Alice and Bebe, but we all now have a responsibility to ensure that something like this never happens again.
“We will be doing all we can to assist the chair through the inquiry and uncover the truth.
“It is only through intense public scrutiny that real change can be effected.”
The inquiry begins at 2pm.
