The three biggest lies in America, 1986 edition: 1. "I gave at the office." 2. "The check is in the mail."

3. "This computer is 100 percent IBM compatible."

The first two of these traditional fibs are so familiar by now that they set off automatic alarm bells for anyone who hears them. This same caution evidently does not apply, however, when computer buyers encounter that ritual incantation, "100 percent IBM compatible." Many people seem to accept with complete trust the salesman's promise that the personal computer they're buying will accept any hardware or peripheral device designed for the IBM-PC and run any software written for the IBM.

Sadly, this promise is rarely true. To be precise about it, in fact, the promise of "100 percent compatibility" never can be true, because there are certain aspects of the IBM-PC design that no manufacturer of IBM clones can legally duplicate. Of course, some computers come extremely close; the Compaq, Kaypro and Zenith clones are famous among computer buffs for running most, but not all, IBM programs without any glitches.

One of the computers that is widely ballyhooed as "100 percent compatible" or "completely compatible" is the Leading Edge Model D, a nice machine that has been aggressively marketed across the country for the past few months. I tested the Model D briefly and found it to be a good computer at a pretty good price: the standard configuration comes with two floppy disk drives, a monitor, serial and parallel ports, and built-in clock/calendar for $1,495 -- or less if your retailer is eager to make the sale.

A few weeks ago in this space I noted that there have been complaints from some Model D owners about compatibility problems -- certain IBM programs and some hardware add-ons for the IBM-PC would not work on the "completely compatible" Leading Edge machine.

Normally, whenever I say anything negative about a computer product, I am deluged with mail and phone calls from indignant readers sticking up for that product. The comments here about the Model D, though, brought a somewhat different response: people started calling up and telling me about various programs that would not run on this machine. One Model D owner told me she would rate the computer about "55 percent compatible."

It's important to note that all the Model D owners who contacted me had nice things to say about the operation and reliability of the machine; but some felt they'd been snookered by that claim of "complete compatibility."

There's another caveat that any potential Model D buyer should keep in mind as well: Some retailers and mail-order houses are advertising a Leading Edge computer that is not the Model D. This impostor -- often available for $1,000 or less -- is an earlier Leading Edge model called the "Model M." It has fewer features than the Model D; further, its IBM compatibility also may be suspect.

Frankly, I wouldn't buy either Leading Edge computer. It's a matter of origins. The Model D is actually built by Daewoo, in South Korea. The Model M comes from the Japanese manufacturer Mitsubishi. As I've often said, Americans should buy American-made computers. For one thing, they're better computers. For another, buying American is a way that American consumers can reward companies that resist the temptation to ship jobs overseas to places like Daewoo and Mitsubishi. Does anybody agree with me on this "Buy American" business? Some folks do -- including, of all people, the folks at Leading Edge Inc. This month Leading Edge is running ads touting its word-processing software. Here's the argument Leading Edge uses to convince you to buy its program: "The Leading Edge Word Processor was created and made in the U.S.A.!"

There's no shortage of sheer, unmitigated gall in the personal computer business, but this may take the cake -- a company that asks you to "Buy American" in the software market at the same time it is staging a massive marketing campaign to convince you to "Buy South Korean" when it comes to hardware. Leading Edge wants you to be patriotic in your purchases -- but not too patriotic.

Frankly, I agree with Leading Edge that computer buyers should look for products "created and made in the U.S.A." If you want an IBM clone, take a look at the Compaq, Kaypro, Zenith or other home-grown machines.

And, of course, if you want a computer that really is "100 percent IBM compatible," there's only one computer that fits the bill: the IBM-PC.