The House Ways and Means Committee yesterday approved $20.9 billion in deficit-reducing steps for the next three fiscal years, including a new tax on federal employes that could cost those in high pay grades as much as $450 next year.

The federal employes would have to pay the Medicare portion of the Social Security tax, which next year will be 1.3 percent of about the first $35,000 of pay. In return they would become eligible for Medicare, which many now get anyway because they also spend time in private employment. The Senate Finance Committee has approved the same new tax, making enactment quite likely.

The Ways and Means panel, acting under the deficit reduction instructions contained in the fiscal 1983 budget resolution Congress adopted last month, also approved new limits on the year-to-year increases in costs for which the government will reimburse hospitals under Medicare; new limits on what Medicare will pay hospitals for such so-called ancillary services as blood tests and X-rays, and ultimately a new system of so-called prospective reimbursement under which a hospital no longer would recover all its costs under Medicare, but would be paid a preset amount for each service it performed.

Together these various provisions, also adopted by Senate Finance in similar form, would constitute a form of partial price controls for medical care.

In addition, Ways and Means, in an unusual session behind closed doors, approved an extra 13 weeks of unemployment benefits for those in high-unemployment states who have exhausted their regular 39 weeks of insurance and an increase of $6 billion over the next three years in the unemployment tax now paid by employers.

In the Senate, meanwhile, the Agriculture Committee, also working to comply with the budget resolution, tentatively agreed on $2.6 billion in cuts in the food stamp program over the next three years to be achieved mainly by delaying the cost-of-living increases to which recipients are entitled under law. This would be the second large food stamps cut in two years and would have been larger had Agriculture Committee Chairman Jesse Helms (R-N.C.) had his way. But Sen. Robert J. Dole (R-Kan.) restrained Helms.

The committee tentatively approved a Helms proposal to let states take the food stamp money that now goes to their residents and turn it into a block grant, which the state governments could then administer to feed poor people as much as it chose. But observers predicted this would be dropped from the bill later in the legislative process; no bill to extend the expriring food stamps program past this year has yet moved in the House.

The Senate Veterans' Affairs Committee also voted budget cuts of about $560 million over three years, about $140 million short of its target, mostly by approving over Democratic objections a 0.5 percent user fee on Veterans Administration home loans.

The Ways and Means markup yesterday was an early test of how the Democratic House can be expected to comply with the budget resolution, which many Democrats resisted as too restrictive. Some observers had expected the Democrats to rebel in this recession-and-election year, and they did yesterday at least force Republicans to take the lead in proposing spending cuts and tax increases.

The committee voted 23 to 3 to conduct its deliberations behind closed doors after Chairman Dan Rostenkowski (D-Ill.) said he thought such a step would help minimize politicking. Ranking Rep. Barber B. Conable Jr. (R-N.Y.) concurred. Aides also said later that no recorded votes were taken in the session, all were voice votes.

Ways and Means has jurisdiction over most of the government's basic spending programs, and hit its deficit-reduction targets for them. It also has jurisdiction over taxes and will write a tax bill later; Senate Finance has approved about $20 billion in tax increases for next year.

The Medicare reimbursement provisions are similar in effect to the hospital cost containment proposals that President Carter failed several times to get through Congress. Rising health care costs have been important factors in inflation and the growth of the federal budget in recent years; the federal government pays nearly 40 percent of U.S. health care costs.