Teancum Vernon Petersen-Crofts was charged with three counts of murder the day after his mother Michelle, 48, and little brother Rua were found dead inside their Ellenbrook home.

His sister Bella was found critically injured in the backyard but died on the way to hospital on Sunday.

Flanked by two security guards and without a lawyer, Petersen-Crofts appeared in person at Midland Magistrates Court on Monday where he was only asked to confirm his identity.

He looked around the public gallery before sitting down and accusing another person of carrying out the attack.

"A criminal wanted to take me out bush - he did it and he came back for my family," Petersen-Crofts said.

The magistrate told him he didn't need to say anything, but he replied: "I have to 'cos mamma is with me."

After the magistrate said Petersen-Crofts would be sent to a psychiatric facility as there were concerns for his mental health, he said: "I was painting a picture of Jesus and stayed there for two months.

"He punched me in the head so hard. I left because I had to. I was that scared he was going to kill me.

"Mamma told me to save myself."

Petersen-Crofts then told a security guard standing by his side that he looked "like a serial killer."

He was remanded to the secure Frankland Centre unit at Graylands Hospital and will face Stirling Gardens Magistrates Court next week.

Police have confirmed he lived at the property with his three victims.

Friends of Bella said they were shocked to hear about the tragedy.

"She would come to school and she would be so happy," a friend told Seven News.

"I just don't know how this happened."

Assistant Commissioner Paul Steel told reporters on Sunday a senior homicide squad investigating officer had said it was "potentially the most horrific crime scene he has ever been called to".

"It is a very complex and I would say horrific crime scene."

Police were initially called about 1.30am to a nearby convenience store before they went to the home.

It's understood Petersen-Crofts had gone to the store asking for help.

As a forensics team combed the area, an officer was seen collecting a pair of scissors from scrub near a roadside and placing them inside a container.

A neighbour said he saw police cars speed to the house after hearing "a bit of screaming and yelling, and a bit of fighting".

Another neighbour said the street was usually quiet.

Australian Associated Press