Fannie Mae, the government-supported mortgage company, raised $6bn selling three-year securities at lower yields relative to government notes than in its last offering of similar debt, as central banks and foreign investors made up the largest share of buyers in two years. The company, in its largest sale of benchmark debt since March, offered a yield of 1.257%, or 23.5 basis points more than similar-maturity Treasuries, Fannie Mae said today in an e-mailed statement.