RCMP Const. Gary O'Brien says officers went to the room Wednesday morning after learning a man and two women staying there may have firearms.
He says a 29-year-old woman was arrested in the room and that a 28-year-old man and a 32-year-old woman were taken into custody on a nearby street.
Police then searched the room and found a grenade launcher, which O'Brien says can be attached to the barrel of many rifles and used to lob grenades a considerable distance.
Investigators and forensics experts are examining the launcher to determine if it is stolen or associated with any other police occurrences.
No charges have been laid as the investigation continues.