Donald Trump carried Pennsylvania by about 72,000 votes; Wisconsin by 26,000 votes; and Michigan by 12,000 votes. If these three states had gone the other way, Clinton would have won. Clinton did not campaign once in Wisconsin during the general election.
(These are initial results. Official certified returns will probably show a different spread.)
Update: Trump won Michigan even though he received 30,000 fewer votes than George W. Bush did in 2004, when he lost the state. This means that voter turnout was less in Michigan in 2016 than in 2004.