Shannon Boxx and Abby Wambach also scored for the Americans, who are seeking their ninth title in 18 appearances in the event.
Iceland nearly scored first with a header from inside the penalty area in the 19th minute, but goalkeeper Jillian Loyden made the save.
Buehlers opened the scoring off a corner by Lauren Cheney corner in the 48th, and Boxx doubled the lead when she headed in Christie Rampone's cross in the 63rd.
Wambach scored her 154th career goal off an assist by Alex Morgan in the 86th, putting her four goals away from tying Mia Hamm's team record.
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