New federal workers would be put into a three-layer system combining Social Security, civil service retirement and a private investment program under legislation to be introduced this week by Sen. Ted Stevens (R-Alaska).

Stevens' bill would not force current federal and postal workers to come into the new system. But those who wanted to could switch to the complex new program. They would join Social Security, keep a modified civil service pension program (which they would not have to pay into) and also be allowed to pay up to 6 percent of annual salary into a privately managed investment fund. Agencies would match 50 percent of employe contributions to the fund.

Unlike the civil service system, where annuities are guaranteed based on salary and service time, benefits under the new system would be a combination of Social Security-civil service payments, plus the value of an employe's "shares" in the matching investment fund.

Hearings on the Stevens bill will begin next week before the Governmental Affairs Committee. The Reagan administration has not decided whether to support it.

On Dec. 11 this column reported that the Congressional Research Service was working on a report for Stevens that could lead to mandatory Social Security coverage for new hires. The CRS report was completed last month. Aides to Stevens, who is assistant majority leader, are putting the final touches on the bill. It will be formally introduced later this week.