If you're one of the estimated 3 million Americans who bought or sold a home last year with the help of "creative financing," you ought to know about a clever tax trap that Uncle Sam is setting for you.

The Internal Revenue Service hasn't announced it, but the agency is about to conduct a nationwide campaign to catch creative real-estate financers who aren't paying their fair share of taxes. The clues to the coming crackdown can be found in the new federal tax forms that you should have received earlier this month.

Tucked away in the two most frequently used attachments to the standard 1040 form are questions and instructions from the IRS that never have appeared before. On Schedule A--the sheet on which you're supposed to itemize your deductions--is a revised question regarding "interest expense."

The IRS asks you not only to declare the amount of home-mortgage interest you paid during 1982 to "financial institutions," but also to "individuals." The query about financial institutions is what homeowners have been answering for years in order to write off part of the cost of their mortgages.

The reference to "individuals," however, is new, and it seeks something special. The IRS wants you to fill out the names and addresses of the individuals to whom you paid mortgage interest during 1982.

In other words, if you bought a home last year with the help of a first, second or third mortgage--or a land contract, wrap-around loan or any other form of creative financing--the IRS wants the identity and street address of your creditor. Even if that financing went unrecorded, and even if it violates the mortgage rules set by a bank or savings and loan association (as in the case of so-called "underground" assumptions of nonassumable home loans), the IRS wants to know about it.

The IRS wants the information badly enough, in fact, that it will deny mortgage-interest deductions to any homeowner who doesn't include the name and address of a "creative financing" lender with the tax forms, according to a source at the agency.

The second unannounced but related change in the form is in Schedule B, the flip side of Schedule A. Schedule B asks for information on "interest and dividend income." A new line in the 1982 form requests disclosure of any "interest income from seller-financed mortgages," and asks the name of the "payer"--i.e., the home buyer you helped to finance.

Here is where the IRS intends to spring its trap on the unwary. Because most buyers of seller-financed homes will want to claim a deduction for the mortgage interest they've paid, the IRS will be able to cross-check returns to see if the sellers are reporting interest income they received.

In other words, the names and addresses on the buyer's Schedule A will be the tip-off to the identity of the 1982 creative financer. A simple computer check of Schedule B forms should flag prime candidates for an audit.

The technical, euphemistic name for the IRS's new seller-financing trap is a national "compliance survey," according to a spokesman for the agency.

In plain language, the new disclosures will be included in a statistical sample of home buyers and sellers across the nation who report use of nontraditional financing. The people in the sample--which should number well into the thousands, according to an IRS official--then will be marked for special analysis.

Home buyers who filled out the new Schedule A deduction form will be asked to submit proof that they did indeed pay mortgage interest to the individuals they named. Obviously, the IRS won't go along with your deductions if your alleged payments turn out to be bogus or can't be documented. That could put you in hot water for tax fraud. If, as the IRS suspects will be the more common problem, your "creative" mortgage-interest payments don't show up on the Schedule B (interest income) disclosure of the seller-financer you named, then your seller will be in hot water.

Part of the objective of the IRS campaign this year will be to "attempt to assess how widespread taxpayer noncompliance is" in the area of creative-mortgage financing, said an IRS source.

"We know there was a hell of a lot of seller financing last year, after all," he said. "We know that something like 60 to 70 percent of all resales involved some type of seller participation--especially when mortgage rates were sitting at 17 and 18 percent. But will those sellers report the income from those transactions? That's what we want to find out."

He warns that, if the IRS finds out that many sellers conveniently forgot to report their creative-financing interest earnings, then the agency might undertake a more thorough series of audits--touching a much higher percentage of taxpayers who bought or sold homes in 1982.

Some practical advice: Don't fool around on this one. The IRS means business.