On April 28, police found the bodies of Kyle Devine, 27, Clarissa English, 24, and Dakota English 18, in a townhouse in the west end of Lethbridge.
Police allege the accused was known to all three victims and had been in their company prior to their deaths.
An autopsy found the three died from multiple stab wounds.
Austin Lane Vielle, who is 21, has been charged with three counts of second-degree murder.
At the time of the deaths, Vielle was out on bail charged with assault with a weapon and uttering threats against Devine earlier in April.
One of the bail conditions was that he have no contact with Devine.
The day after the deaths, police arrested Vielle and charged him with breaching his conditions.
On Wednesday, police arrested Vielle at the Lethbridge Correctional Centre and charged him Thursday in the deaths.
He remains in custody and will appear in court in Lethbridge on May 21.
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