A study of 483 multi-employer pension plans with 3 million participants found that a large majority are in excellent financial condition, with 73 percent fully funded for all vested benefits and another 14 percent funded at 80 percent or more for vested benefits, Martin E. Segal Co. reported yesterday.

Only 3 percent were less than half funded, said Segal President Robert Krinsky.

Segal is one of the biggest pension and actuarial consulting firms in the country. The 483 plans covered in the survey were all Segal clients and they were all traditional defined-benefit plans in which the employer or fund guarantees a certain level of benefits. They constitute one-quarter of all multi-employer plans in the nation, and their 3 million participants make up more than one-third of all workers covered by multi-employer plans in the United States.

Multi-employer plans are one of two basic types of private employer pension plans in the United States. The other are single-employer plans.

In a typical single-employer plan, a firm has one pension plan covering its own employes and the plan is managed by the firm.

In a typical multi-employer plan, such as in the construction industry, a number of employers in the same industry join with the union covering that industry to set up one plan to which all the employers contribute. The plan is managed by a joint labor-management board. Workers shifting from one employer to another within the industry or skill-grouping stay in the same plan.

According to the U.S. Pension Benefit Guaranty Corp., there are about 29 million workers in single-employer plans in the United States and 9 million in multi-employer plans.

Krinsky said the study, the third released by Segal in the past three years, shows that, despite fears, "multi-employer plans are better funded than often assumed and that their funded position, relative to vested benefits, has remained exceptional." He said he believes that the "well-funded" position of the firms surveyed by Segal is "pretty representative" of the whole range multi-employer plans in the nation, although there are clearly pockets of trouble here and there where an industry may be declining and therefore having trouble with financing.

According to the Segal survey, the 483 plans had $27 billion in combined assets in 1985. Seventy-three percent of the 483 were fully funded for vested benefits, which was about the same proportion as last year, 14 percent of the plans had funding of 80 percent or better, 9 percent of the plans were between 50 and 80 percent funded, and 3 percent were under 50 percent funded.

Krinsky said the plans in the Segal survey with low funding ratios are in "no danger whatsoever" of collapsing. Typically, the low-ratio plans in the Segal group consisted of new plans that started out with no assets but sometimes had picked up liability for retroactive benefits for existing employes and simply needed time to build up funding levels, or older plans that had granted retroactive benefit increases that were not yet fully funded.