WASHINGTON (AP) _ Guidance systems on the MX nuclear missile don't work properly because Northrop Corp. was more interested in making money than it was in building proper computer systems, three current or former employees of the defense contractor told Congress on Friday.

The hearing by two House Armed Services subcommittees was the latest round in an investigation into widely publicized problems with the guidance system for the 10-warhead MX.

The guidance systems, known as inertial measurement units, or IMU, are so fouled up that one-third of the 21 MX weapons now in ground-based silos are not operational, according to previous testimony.

The guidance systems were constructed by Northrop's Electronics Division for the Air Force to put atop the MX, which has long been one of the most controversial weapons in President Reagan's defense buildup. Reagan originally wanted 100 of the weapons, but after a long fight, Congress cut the program to 50 missiles.

Bryan Hyatt testified that he worked for Northrop as an engineer for five years until he said he was fired last year for telling the Air Force that there were problems in the MX guidance system.

''It's a case of management run amok,'' said Hyatt. ''Their interest is greater in delivering the IMUs to the Air Force than in delivering them to the Soviet Union.''

Later, Hyatt said the guidance systems were so fouled up that ''these missiles stand as much chance of hitting their targets in the Soviet Union as they do of landing here in Washington, D.C.''

Hyatt's contentions were supported by Jeff Kroll, who is currently a manager at Northrop, and Dave Peterson, who worked as an engineer for the company until last spring.

When Northrop first received the contract for the systems, according to Peterson, the company didn't properly ''base-line'' the production equipment, meaning that thousands of engineering changes were eventually needed to make the systems work.

Because of all the changes, Peterson said, production fell behind and Northrop eventually decided to set up a dummy company, Liaison Engineering Services Inc., to handle procurement of the needed parts for the changed systems. Peterson was one of the people picked to run the dummy firm.

Kroll said the ''problems related to the procurement of parts arose out of Northrop's failure to properly design equipment prior to production.''

He said ''there have been threatening statements'' made to him and his wife by company officials.

John Thorne, a spokesman for Northrop, said: ''These are serious allegations and we need to wait until all the facts are in before a judgment is made. Some have already turned out to be false and on others, Northrop has already taken corrective action.''

He noted that Northrop officials will appear before the subcommittees next week to tell their version of the facts.

As for whether the specific allegations were true or false, Thorne replied, ''There is no way I can answer that.''

The Air Force, in a prepared statement released late Friday, agreed it was dissatisfied with Northrop's ''rate of delivery'' for the inertial guidance units needed for the missiles.

But the service also flatly dismissed suggestions it had any concerns ''about quality or performance.''

''The IMU's performance has never been in question,'' the Air Force said. ''There have been 17 flight tests and 17 successes, all highly accurate.

''The accuracy right now is considerably better than the specified requirement for three-to-five years from now. And during eight months of operation at F.E. Warren Air Force Base, we have had half as many failures to date as expected.''

The Air Force said it was true that only 14 of the 21 MX missiles now deployed in silos were ''operationally ready,'' but added that reflected a conscious decision to divert some available guidance units to maintenance training and to maintain ''spares to support the 14 missiles.''

''Sufficient IMU's are available to bring all 21 missiles to alert should the need arise,'' the Air Force said. ''And we have high confidence in meeting next major milestone, which is full operational capability in December 1988.''