Organizations fighting to preserve a pension tax benefits for 20 million government and private-sector employes hope to win one of three points in the ongoing Senate-House conference on the tax reform bill.

Their primary goal is to persuade Congress to leave the so-called three-year recovery rule untouched. That regulation permits people who've contributed to their pension plans to get back all previously taxed contributions before their annuities are subject to federal tax.

The recovery rule benefits federal retirees, millions of state and local government aides and others who contribute to their own retirement programs. The typical federal retiree gets back all pension contributions in about 18 months, and then begins paying taxes on the money the government contributed to the pension plan.

Tax reformers are considering eliminating the recovery rule to require persons to pay taxes as soon as they retire, on a prorated portion of the government contribution based on each individual's life expectancy. The change wouldn't increase the retirees' tax burden over a normal life span, but would require them to begin paying taxes much sooner.

If conferees insist on eliminating the recovery rule, lobbyists will then ask that the change be made effective in the future, rather than retroactively. The House tax reform plan would eliminate the recovery rule retroactive to the first of this month. The Senate plan would phase in the change over a two-year period, beginning in January 1988.

If the recovery rule is eliminated, lobbyists will ask tax bill writers to protect a benefit signed into law June 6 that allows retirees to get one-time, lump-sum, tax-free payments equal to the amounts they have contributed to their pension funds.

That one-time payment was written into the new federal retirement law to soften the impact of loss of the recovery period. The payments would be worth about 18 months of pay for the typical retiree.

Although the new benefit is in effect, the government cannot issue instructions to agencies on how to handle it -- and prorate taxes on the remainder of the pensions -- until the Senate-House conference reaches agreement on a tax reform bill. One provision of the Senate tax plan would eliminate most or all of the tax advantages of the new benefit, hence the delay in instructions to retirees, employes and agencies.

Meantime, the Federal Government Service Task Force, a congressional civil service caucus, is making a last-ditch appeal to all members of Congress on behalf of the three-year recovery rule. The task force says the benefit should be retained, but if it is taken away it wants to protect the new lump-sum payment system from being altered by tax reform.