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By David Ljunggren

OTTAWA, Dec 5 (Reuters) - Three Canadian soldiers were killed by a massive bomb in Afghanistan on Friday, bringing to 100 the number who have lost their lives since the country's military mission there started in 2002, officials said.

Canada has around 2,700 soldiers in the southern city of Kandahar on a combat mission that is set to end in 2011. One diplomat has also been killed.

"Already there is talk of numbers and milestones but it is my hope that the focus remains on the lives and sacrifices of these brave soldiers," said Brigadier-General Denis Thompson, who commands the Canadian force in Afghanistan.

The men died when their armored vehicle rolled over an improvised explosive device (IED) west of Kandahar.

"It was a very large IED. It killed them all instantly ... you have good days and you have bad days. It just so happens that today was a particularly bad day," Thompson told a televised news conference in Kandahar.

He named the dead men as Corporal Mark McLaren and Private Demetrios Diplaros. He did not identify the third man.

In a separate incident in Kandahar province on Friday, two Canadian soldiers were seriously injured when a bomb went off near their foot patrol.

Prime Minister Stephen Harper told troops at a military base in Ontario that the military had suffered "three terrible losses".

Thompson said Canadian troops would stick to their plan to keep on the offensive against Taliban militants this winter.

"This is a risk that we want to take in order to make sure we maintain the initiative and keep the insurgents off balance so next summer's fighting season isn't ... as rigorous as last summer's fighting season," Thompson said.

Canada has long complained that its troops have to bear a disproportionate burden of the fighting and wants other NATO nations to contribute more.

The Canadian mission was initially supposed to end in 2009 but Parliament agreed to an extension to 2011 as long as the troops shifted their focus away from fighting and more toward helping the reconstruction effort.

"I believe that that change hasn't happened, isn't about to happen and that's what we've been arguing for," said Ujjal Dosanjh, defense spokesman for the opposition Liberal Party. (Reporting by David Ljunggren; editing by Peter Galloway)

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