It is strange enough that the state of Idaho has three state fairs. Stranger yet is that the people of Idaho find nothing strange at all about this tripartite state of affairs.

"If you lived here," said David O. Porter, head of the state's Economic Affairs Department and a leading political guru, "you'd know that Idaho has to have three fairs. We've got no choice, you see, because in a sense there are three different Idahos."

"Idaho never did make much sense as a geographic unit," agreed Arthur Hart, of the Idaho Historical Society. Ever since this L-shaped hunk of craggy mountain and crusty plain was admitted to the union in 1890, Idahoans have been battling one another with a vigor that makes the Balkan states look like bosom buddies.

Contrary to what the maps show, Idaho is really three states with three capitals.

The arid, agricultural southeastern corner, where some counties are nearly 90 percent Mormon, is essentially an extension of Utah; its capital is Salt Lake City.

The lushly forested northwestern arm, populated by fiercely independent free-thinkers ranging from the nation's largest band of neo-Nazis to Taoists descended from the Chinese who came here to mine silver and lead, revolves around Spokane, Wash.

In the middle, paying fealty to Boise, the official state capital, lies Idaho's "fertile crescent," the serpentine Snake River valley blessed with a black volcanic soil that produces nine of every 10 McDonald's French fries.

These disparate Idahos are divided by mountain chains that make travel difficult and by ideological, economic and religious differences. The three-way split fuels strange political alliances -- which may well cost the Republicans a U.S. Senate seat next year -- as well as a secession movement that is 100 years old and going strong.

For many people here, however, the different Idahos emerge most clearly in these crisp, dry days of early September when Americans -- Idahoans and otherwise -- head out to the state fair.

The United States in 1985 is overwhelmingly an urban/suburban nation, with less than 10 percent of the population in rural areas. Yet the state fair, that antique celebration of rural life and values, is flourishing as never before.

Every state but Massachusetts, Pennsylvania and Connecticut has an official fair. In most places, the annual end-of-summer state fete is a great unifier in which farm folk and city slickers join to honor their common heritage.

Not so in Idaho.

The state fair season this month finds Idahoans, as usual, going their separate ways to the Eastern Idaho State Fair in Blackfoot, the Western Idaho Fair in Boise or the Northern Idaho Fair in Coeur d'Alene.

The eastern fair is dominated by agricultural exhibits -- livestock, grain, and vegetables. Many exhibitors list not only their name and home town but also their "ward," or Mormon parish.

The northern fair is a teeming tribute to the mining and timber industries. There is no visible Mormon presence, but Buddhists and Bahais are on hand distributing their flyers.

The Boise fair has gone urban. The race track and the agriculture building play second and third fiddle to the huge midway and the long row of junk-food stands that line the main thoroughfare of the fairgrounds.

And what is the most popular food at the biggest fair in Idaho? You can buy baked potatoes, stuffed potatoes, "tater tacos," "tater pigs," French fries, carnation fries, curl fries and swirl fries, not to mention a culinary rarity called a "Spudnut" -- a glazed doughnut stuffed with mashed potatoes.

(Throwing caution to the wind, your intrepid correspondent tried a Spudnut: pretty good, but not as good as the tater pig.)

Most Idahoans stick to their own fair and shun the other two. The exceptions to this rule are the politicians, who love to complain that they have to work three times as many state fairs as their counterparts elsewhere.

With some deviations, Idaho has generally been a populist Democratic state in the libertarian northwest and conservative Republican in the Mormon southeast, with the geographic middle falling somewhere in the political middle as well.

In that situation, the ideal statewide candidate would seem to be a moderately liberal Democratic Mormon, who can bridge the political and religious divisions. The state's current governor, John V. Evans, fits that description. Democrats, consequently, were thrilled when Evans agreed to run next year for the Senate seat held by Steve Symms, a conservative Methodist Republican who rode the Reagan tide to victory over the late senator Frank Church in 1980.

But some northerners want nothing to do with downstate politicians, and there is a strong and surprisingly serious secession movement.

The most popular plan calls for the formation of a 51st state in the rectangle reaching from Walla Walla, Wash. to Missoula, Mont. The idea has won broad support in opinion surveys in the region and is supported by several prominent politicians in Idaho, Washington and Montana.

There is no agreement yet, but leading candidates include Washtanaho, Idawashaho, Idawanna and Columbia.

The secessionists do agree, however, on a their proposed state's motto.

Of all the things northern Idahoans find appalling about their downstate brethren, the most offensive is the motto on the Idaho license plate: "Famous Potatoes." Secession proponents have devised a riposte for the plates their state will issue:

"Washtanaho: No Small Potatoes."