The Treasury said yesterday that it will need to raise about $18.75 billion in new money during the last three months of this year and said it would raise about $4.1 billion of that through note and bond sales next week and the week after.

Assistant Treasury Secretary Roger Altman said that including the $4.1 billion financing announced yesterday, the Treasury will have raised $8 billion, "leaving another $10.75 billion to be done" by the end of December.

The agency expects to have a balance of $12 billion to end the year.

The agency will sell $6.5 billion in securities in three separeate auctions to raise $4.1 billion in new cash and refund $2.4 billion in maturing securities.

In its regularly quarterly financing - held earlier than normal because of Secretary W. Michael Blumenthal's Mideast trip, the agency will sell:

$3.25 billion of three-year notes on Oct. 28. The minimum denomination will be $5,000.

$2 billion of 10-year notes on Nov. 1. The minimum purchase is $1,000.

$1.25 billion of a new 30-year bond on Nov. 2. The bond will be callable after 25-years and will be sold in minimum denominations of $1,000.

Altman said the agency expects to be short of cash in November before it receives the proceeds of the $6.5 billion quarterly financing operation. As a result, Altman said, he expects the agency will announce short-term cash management bills within two weeks to tide the government over the low-cash period.

Altman said the agency is planning its cash-raising operations on the assumption that the federal deficit will be $61.5 billion in fiscal year 1978, which began Oct. 1.

He said it now looks as if the deficit in fiscal 1977, which ended Sept. 30, will be close to the administration's Aug. 1 estimate of $45.7 billion. While the 1977 deficit is well below the February estimate of $68 billion, some analysts had thought it would be as low as $41 or $42 billion. The final figures on fiscal 1977 will be available next week.