Last year, the aid agency raised only 52 per cent of its $2 billion target, forcing cuts to programs for Syrian refugees and other children in crisis.
David Morley, the UNICEF Canada president, says the four-year-old Syrian civil war and other crises in Iraq, the Central African Republic and South Sudan are straining resources to an unprecedented extent.
The UN says it needs to help 62 million children in 71 countries who are at risk in humanitarian crises.
Morley says the unexpected outbreak of the Ebola virus in West Africa also caught the international humanitarian relief system off-guard last year.
Canada is UNICEF's seventh-highest donor and the agency is in talks with the Harper government to contribute to this latest funding campaign, which begins today.
"It feels audacious perhaps to be asking for $3.1 billion," Morley said, but he added that the operating budget for the city of Montreal is $4.9 billion while the Toronto's is closer to $10 billion.
UNICEF won a major victory for embattled children earlier this week when it brokered the release of 3,000 child soldiers in South Sudan. The first 300, boys aged 11 to 17, were released Tuesday.
Morley said it was the largest-ever demilitarization of child soldiers.
"I don't know if it's a coup for us, but it's a coup for the kids, absolutely."
The boys were among 12,000 child soldiers forced into combat in South Sudan, an oil-rich country where fighting has forced 1.5 million people from their homes.
Last month, UNICEF released a report that called 2014 a year of "horror, fear and despair" for an unprecedented number of children in the world's war zones.
A UNICEF specialist on the conflict in Central African Republic — where an estimated 10,000 children were forced to become soldiers last year — was in Ottawa this week for talks with the Harper government .
Judith Leveillee, the deputy UNICEF representative for the Central African Republic, said she got no new commitments from officials at Foreign Affairs, but based on Canada's past support she hopes more will be forthcoming.
Last year, Canada contributed about $5 million to three agencies working in that country, including UNICEF.
"It's a forgotten crisis," Leveillee said of the Central African Republic. "It's really important to sensitize the public, to sensitize government and mobilize resources where we can really make a difference."
About 5,000 people have been killed there in a year of fighting between a Christian militia and a Muslim rebel coalition that took over the country.
Leveillee said aid workers have been able to reach remote areas of the country to distribute relief supplies and run a vaccination project that helped curb a potential polio outbreak among refugee children. But there's still much to be done.
"In the last two years, children have not been able to go to school, they have not been able to access health services. It's been extremely difficult."