Byelections will be held on Nov. 26 in the ridings of Victoria in B.C., Calgary-Centre in Alberta, and Durham in Ontario, the prime minister announced Sunday morning.
The announcement of the three contests came as NDP Leader Tom Mulcair was giving a speech to party faithful gathered in Ottawa for a meeting of the party's federal council.
Speaking to reporters after the speech, Mulcair said his party would "fight hard" in all three byelections.
"It's the only way I know how to do politics," Mulcair told reporters. "I don't concede anything to an adversary ever."
The ridings became vacant when three members of Parliament resigned their seats going back to May and as recently as this summer.
Deputy Speaker and NDP MP Denise Savoie announced her resignation in August, citing health reasons.
The Ontario riding of Durham was left vacant with the resignation of embattled International Cooperation Minister Bev Oda last July, following controversy over her spending habits, including an incident in which taxpayers had to foot the bill for a $16 glass of orange juice during an overseas work trip to London.
Oda eventually paid taxpayers back for a portion of the trip once details of her expenses were revealed.
Conservative MP Lee Richardson stepped down in May to take a job working as Alberta Premier Alison Redford's principal secretary, leaving the seat in Calgary-Centre vacant.
Conservative spokesperson Fred DeLorey told CBC News the party's candidates "will be contrasting the strong economic record" of the prime minister with "the dangerous economic policies" of the NDP leader.
Competing for a seat in Victoria are Conservative Dale Gann, NDP Murray Rankin, Liberal Paul Summerville and the Green Party's Donald Galloway.
New Democrats have held Victoria since 2006 and Rankin, who was in Ottawa for Mulcair's speech to party brass, told CBC News "it's their values that we are fighting back against."
"I'm going to be going against the Conservatives," Rankin said.
While Conservatives are hoping former journalist Joan Crockatt will win the seat in Calgary-Centre, DeLorey downplayed expectations saying "majority governments don't win by-elections."
In a written statement to CBC News on Sunday, Crockatt said "I am taking nothing for granted."
"I am going to keep working hard meeting voters in Calgary Centre and encouraging them to get out to vote Conservative."
Liberal Harvey Locke and the Green Party's Chris Turner will be vying for the seat in Calgary-Centre. The NDP has yet to nominate a candidate there.
The candidates running in Durham are Conservative Erin O’Toole and Liberal Grant Humes. No candidate for the NDP or the Greens has been nominated yet.
In a telephone interview with CBC News, Humes said the benefit of a byelection is "it's not going to have the same national media conversation that goes on with a general election. I think we can actually have debates about issues."
Ruling on Etobicoke Centre coming Thursday
On Thursday, the Supreme Court of Canada will decide whether there should be a byelection in the Toronto riding of Etobicoke-Centre which is currently being held by Conservative MP Ted Opitz.
In a telephone interview with CBC News on Sunday, Nik Nanos, pollster and president of Nanos Research, said he isn't surprised Harper didn't wait for that ruling before calling these three byelections.
"If they rolled up that particular byelection with the other ones, they would have to worry about a contamination effect, right?"
This way, "the group of byelections become a bit of a lightning rod" for whatever the Supreme Court decision is, Nanos said.
Last spring, the Ontario Superior Court overturned Opitz's win after a legal challenge by former Liberal MP Borys Wrzesnewskyj, who contested the results by citing errors and missing paperwork.
Wrzesnewskyj lost Etobicoke-Centre by 26 votes.