Most of the government's 1.3 million white-collar workers are in line for at least two across-the-board pay raises next year, the first in January with a follow-up increase due in October. In addition, a sizable number of federal workers -- perhaps as many as half a million -- will also qualify for a third pay raise, worth about 3 percent, in 1988.

But before you rush out to buy a house or car based on the assumption that you will get three raises next year, consider this scenario:

Raise One: Almost a sure thing. White-collar workers are all but guaranteed some kind of raise in January. The choice is between the 2 percent, $1.5 billion increase the president has proposed and a 3 percent raise that Congress is likely to approve.

Raise Two: Several years ago Congress switched the effective date of federal pay raises from January to October. That legislation expires this year. Unless Congress extends it or allows the president to do so it will mean a return to the old cycle so that in addition to the January 1988 raise, federal workers also would be due an increase in October 1988, the start of the new fiscal year. If there is an October increase, it could be more generous than the usual 2 percent to 3 percent amount because Congress tends to loosen up in election years.

Raise Three: At least three of every 10 federal workers next year will come due for a within-grade step increase. Those raises are worth about 3 percent and are based on a satisfactory job rating from supervisors. The Office of Personnel Management says that 99 percent of all federal workers who come due for the increases get them, so while they technically are not automatic, most people get them. Workers in the first three steps of their pay grade qualify for a within-grade raise every year. Those in steps 4, 5 and 6 of their grade qualify every two years. Those in steps 7, 8 and 9 qualify every three years. Employes at the top of their grade do not get the step increases.Pension Losers

Chambers Associates warns that many federal and postal workers stand to lose major pension benefits by staying with the old civil service retirement system. Between now and Dec. 31, employes who are covered by the old CSRS have the one-time option to switch to the new Federal Employees Retirement System. FERS offers benefits based on Social Security, a modified federal annuity and earnings from investments in the tax-deferred thrift savings plan. So far fewer than 2 percent of those workers eligible to switch to the new pension program have done so.

Chambers Associates is one of several firms here that, for a fee, provides computer data showing workers what their benefits would be under the old and new pension plans.

The consulting firm says that in 30 percent of the cases it has reviewed workers would be financially better off in retirement if they moved into the FERS program even if they made no contribution to the savings plan. If the same group contributed 5 percent of pay to the tax-deferred program, Chambers says, 54 percent to 57 percent would be better off under FERS. The study is contained in an $8.95 booklet available from Chambers. For information call 857-0686.