The Army has mismanaged its controversial training exercises in Latin America for the third straight year, leaving troops poorly trained, equipment unused and millions of dollars wasted, newly released reports show.

The annual road-building exercises, dubbed "Blazing Trails," have long been the target of congressional critics who accused the Reagan administration of staging the massive exercises to intimidate the Marxist Nicaraguan government.The Army Audit Agency reported that the 1987 exercises, which involved 16,000 reservists in Honduras and Ecuador, were plagued by problems similar to those found in exercises of the previous two years.

The May audits said the Army assigned at least 2,000 soldiers to the wrong jobs, denying them training. Truck drivers were used as carpenters and plumbers, cooks worked as truck drivers and construction machine operators, and mechanics and machinists were used as cooks, the reports said.

Auditors also found that tons of Army equipment sat idle in Ecuador because it had no practical use. Dump trucks, once loaded, sank into the soil at a construction site and could not be used. Other equipment remained in port because it could not be transported across the mountains.

Auditors did not estimate the cost of the latest exercises, although the smaller 1986 exercises cost $38.3 million. The office of Rep. Barbara Boxer, D-Calif., quoted Army officials as saying they could not come up with a cost estimate because no central budget authority monitored the exercises.

Boxer, a critic of the exercises who released the two reports to United Press International, said: "I am disappointed and disturbed to receive new evidence that the Army has continued to mismanage and waste taxpayers' funds in military training exercises."

The Army and Defense Department did not respond to requests for comment.