The votes — in Victoria, Calgary-Centre and the Ontario riding of Durham — will be held Nov. 26 and have been called because MPs in each riding stepped down for various reasons.
The most high-profile vacancy in Durham, where former cabinet minister Bev Oda quit following controversy over her expenses, including an infamous $16 glass of orange juice.
New Democrat Denise Savoie resigned her Victoria seat for health reasons and long-time Conservative Lee Richardson quit in Calgary-Centre to work for Alberta Premier Alison Redford.
The call came just as Mulcair was attempting to rev up the NDP machine at a meeting of the party's federal council, which is already plotting strategy for the next general election in 2015.
He found the timing of the prime minister's announcement curious, given that the Supreme Court of Canada will rule this week on whether Conservative MP Ted Opitz will continue to serve as MP for Etobicoke Centre.
"We were expecting the call soon, but we're a little bit surprised," Mulcair told reporters Sunday. "If he'd waited four days we could've at least had that information from the Supreme Court."
Last spring, the Ontario Superior Court overturned Opitz’s win in the May 2, 2011 election after a legal challenge by former Liberal MP Borys Wrzesnewskyj, who argued Elections Canada workers counted at least 79 votes that couldn’t be validated. The Supreme Court will rule on Thursday and Wrzesnewskyj has been out knocking on doors preparing for what he expects will be a byelection.
Mulcair, who took over as NDP leader last March following the death of Jack Layton, said his party intends to turn each of the current byelections into a referendum on the Harper government's record, even in the Conservative heartland of Calgary.
"We're going to fight hard in all of these byelections. It's the only way I know how to do politics. I don't concede anything to an adversary — ever," he said.
New Democrats say they've started their organizational spade work for 2015 and Mulcair used both the meeting and the byelections as a chance to frame the differences between his party and the Conservatives.
"When Canadians go to the polls in 2015, for the first time in a very long time, they'll have a clear choice between two parties with two clear visions for a our country's future," he told delegates, adding later to reporters: "We're already starting to see a choice and that's what is going to be reflected in these three byelections.
In a bid to underscore the perceived differences, he attacked the Harper government for a lack of transparency on key decisions, such as the rejection of a $6 billion bid by Malaysia’s Petronas to take over Calgary natural gas producer Progress Energy Resources Corp.
The decision was released at 11:57 p.m. on Friday night.
"Who releases such an important decision at midnight on a Friday? Someone who has something to hide and no way to explain it," he said.
Petronas will have the opportunity to refile its bid, but Industry Minister Christian Paradis has said the proposed investment as written was not likely to be of net benefit to the country.
New Democrat Murray Rankin, a B.C. lawyer who is running in the Victoria byelection, says the Harper government's approach is making Canada "the Wal-Mart" of the natural resources sector.
He said people, particularly in B.C., don't understand why the refining of resources can't be done at home.
"We seem to be selling our resources out to the lowest bidder," said Rankin. "People think that is just utter folly and they don't understand why we aren't capturing those jobs in Canada."
Also on HuffPost