The Treasury Department has suspended the sale of U.S. savings bonds for the third time this year because President Reagan has not signed a bill raising the national debt limit. The department said it had notified banks and other financial institutions to stop selling the bonds until further notice. Sales were suspended for brief periods in July and August because Congress failed to approve a measure to increase the government's borrowing authority. The Senate gave final approval Wednesday to a measure to raise the debt ceiling to $2.8 trillion and sent it to Reagan. However, the President has not decided whether he will sign the bill because of a separate provision that provides a new enforcement mechanism for the Gramm-Rudman deficit-reduction law.