Oops! That's three millionaires too many!

The slip of a pencil by a US Census Bureau number cruncher gave Alpine County , Calif., three millionaires who don't exist.

This mistake also gave the county - a sparsely populated ranching-cum-ski-resort area in the Sierra Nevadas - the third highest per capita income in the nation. But the fact is, the county's income ranking is closer to 275.

''We knew it wasn't right all along,'' says Alpine County clerk Joan Chacon. But it was the Census Bureau, she says, which discovered its own error.

It took 15 statistical clerks three days to sift through the 1979 income figures gathered for the 1980 US Census, culling out what insiders call ''bogus millionaires.'' These millionaires are statistical aberrations, not people, and they point to some of the difficulties in tallying and interpreting per capita income estimates.

Investigators found figures from some census questionaires had been incorrectly coded by employees onto forms fed directly into federal computers. For instance, numbers that should have been $9,990 suddenly leapfrogged to $999, 000. Officials, sensitive about the error, refuse to say how many bogus millionaires they've unearthed.

In the case of Alpine County, ''somebody miscoded two numbers from the same form,'' says Dan Burkhead, a Census Bureau economic statistician. ''That made this one guy earning in the neighborhood of $2 million.''

Since the census is done as a sample of the total population, the ''millionaire'' was counted three times, pumping about $6 million in income into a county with 1,100 people. When experts discovered the error recently, Alpine County's per capita income was rolled back from $12,861 to $7,669.

To calculate per capita income, the Census Bureau surveys 3,137 counties, county equivalents, or independent cities with 100 or more residents. Incomes are added together and then divided by the number of people in the region.

So three millionaires in a scantly populated area such as Alpine County can skew the final estimate by thousands of dollars. The same three millionaires in a much more heavily populated region would only have minor impact, since their high incomes would be averaged out over many more people.

Less dramatic adjustments have been made in other regions as a result of the coding errors. Per capita income for Fairfax County, Va., for instance, has been reduced $2 since the figures were released several weeks ago.

''The problem wasn't just restricted to millionaires, either,'' says Gordon Green, chief of the Census Bureau's population division. There are also a variety of more subtle slip-ups, such as $1,000 getting recorded as $10,000 or $ 100,000.

As a result, the Census Bureau will review all computer-coded income figures taken from the 1980 headcount. They'll be looking for individual cases where income figures seem unusually high, then matching forms against the coded figures. That task is expected to take at least six weeks, and it's likely to be January before new final estimates are released.

''We're talking about 37 million questionaires with income figures, and it would be miraculous if there weren't any coding problems,'' says Mr. Green.

But while coding problems are blamed for the current difficulties, experts say there are other reasons to question the usefulness of these per capita income figures.

''These numbers, as now calculated, overlook differences in the cost of living in different areas,'' says Kevin McCarthy, senior population analyst at the Rand Corporation. So even though per capita income may be high, it doesn't necessarily mean residents are better off. For example, eight of the top-15 -ranked income areas in the country - which have been checked for bogus millionaires - are in Alaska.

''But it's very difficult in these areas to talk about per capita income in a way relevant to the situation in a typical American community,'' says Mr. McCarthy. The cost of living in Alaska averages from 26 to 50 percent above the national average, so pay scales are naturally higher.

Another complicating factor is the composition of the community. Many of the top-rated income areas are in the suburbs of Washington, D.C., where government employees live. Incomes for these workers are traditionally high.

Per capita income is a hot issue because the US Treasury Department's Office of Revenue Sharing uses the number to divvy up federal funds. The lower the income figure, the more likely an area is to qualify for federal help.