According to prosecutors, the 13-year-old came up with a plan to kill Mironovas by crushing prescription pills and mixing them into a glass of wine, but court documents show the boys' attempt to secretly mix in the pills failed.
Mironovas' son and his friend entered her bedroom wearing gloves and armed with a knife early Sunday morning, according to prosecutors.
Mironovas was found shortly after 2 a.m. Sunday, and police say she had been strangled and stabbed in the neck.
All three boys were arraigned at a juvenile court in Maine on Monday.
The 15-year-old suspects were charged with one count each of murder, and all three were charged with criminal conspiracy to commit murder.
Mironovas and her son had moved to Maine from their home in Ashland, Massachusetts, where former neighbors told Boston 25 News they hope her son gets help.
"She was a quiet person," said Jim, a former neighbor. "I think her son was troubled a little bit, she had him in a special school."
Mironovas moved from Ashland to Maine about a year ago to attend cosmetology school.
Authorities continue to investigate the case.