"Three amigos" summit dominated by swine flu, trade

* Economic slump felt in all three countries

* No unity on simmering trade issues

* Shared commitment to halt spread of H1N1 virus

GUADALAJARA, Mexico, Aug 10 (Reuters) - Leaders of the United States, Mexico and Canada gather on Monday to present a united front to try to limit the spread of the H1N1 swine flu, but there is less unity on simmering trade issues.

U.S. President Barack Obama, Mexican President Felipe Calderon and Canadian Prime Minister Stephen Harper enjoyed folk dancing and a mariachi band late Sunday and now are getting down to business at a North American summit.

They hold talks with all three feeling the heat from an economic downturn.

The leaders, sometimes referred to as "the three amigos," are expected to issue a joint communique stressing their shared commitment to keep a predicted resurgence in the H1N1 virus this autumn as limited as possible.

A senior Obama administration official said the goal was to ensure that the people of the three countries are fully informed about steps to mitigate the spread of the virus, which is believed to have originated in Mexico last spring.

Obama held 45 minutes of talks with Calderon before they were joined by Harper for a dinner at the sprawling Cabanas Cultural Center. Crowds lined Obama's motorcade route.

Calderon wasted little time in raising what Mexico considers a pressing trade issue -- a cross-border trucking dispute.

Mexican trucks are supposed to be allowed to cross into the United States under the North American Free Trade Agreement, but U.S. trucking companies say Mexican trucks are not safe.

Mexico imposed retaliatory tariffs of $2.4 billion in U.S. goods in March after Obama signed a bill canceling a program allowing Mexican trucks to operate beyond the U.S. border.

Obama made clear to Calderon that he was working with the U.S. Congress to resolve what he considers to be legitimate safety concerns.

"I think there's been a clear understanding that this issue was one that is a priority issue and one that everyone would like to see resolved as quickly as possible," the senior Obama official said.

Canadian officials are expected to raise concerns about "Buy American" elements of a $787 billion U.S. economic stimulus program that they fear could shut out Canadian companies.

Canada is the United States' largest trading partner.

Mexico's ambassador to Canada, Francisco Barrio, said that in talks with Harper Calderon expressed concern about a measure to require Mexicans to have visas to visit Canada.

Obama and Calderon also talked about Mexico's warring drug gangs. Washington is worried about the rising body count as Mexican gangs kill rivals in record numbers, despite Calderon's three-year army assault on the drug cartels.

Mexico says U.S. equipment and training under a $1.4 billion "Merida Initiative" package are taking too long to arrive, partly due to concerns in the U.S. Congress over Mexico's human rights record.

Democratic Senator Patrick Leahy of Vermont has blocked what was described as a favorable U.S. State Department report on Mexico's human rights record, delaying the release of $100 million in U.S. aid intended to help Mexico fight narcotics traffickers

Mexico's ambassador to the United States, Arturo Sarukhan, told reporters "there is one senator that is worried about the issue," and that "this is not part of a wide movement inside the Senate." (Additional reporting by Mica Rosenberg, editing by Chris Wilson)