Only three of the nation's 37 major freight railroads earned an "adequate" rate of return on investment last year under standards established by the Interstate Commerce Commission, the agency said yesterday.

Under ICC standards, a railroad is considered to have earned adequate revenue if it had a rate of return equal to the current "cost of capital," defined by the agency as the rate of return a railroad must be able to offer investors to attract capital through the sale of securities.

Last year, a railroad would have had to earn at least a 12.1 percent return on investment to have achieved an adequate return, the agency said yesterday, a standard met only by three of the smaller railroads.

The Fort Worth & Denver Railway Co., a subsidiary of the larger Burlington Northern Inc., led with a 19.3 percent return, followed closely by the Clinchfield Railroad Co., an affiliate of the Family Lines System subsidary of CSX Corp., with a 19.1 percent return. The Pittsburgh & Lake Erie Railroad, with a 17.8 percent return, was third.

Although the ICC report shows few railroads earning what the agency considers adequate revenues, the railroad industry's fortunes do seem to be improving. In an earlier report, the Association of American Railroads reported that railroad earnings rose in 1980, providing the industry with its highest rate of return on net investment in 36 years.

While the AAR report showed the industry as a whole at a less-than-sensational 4 1/4 percent rate of return on net investment in 1980, that compared with a 2.91 percent rate the year before and less than 2 percent for each of the prior four years. The industry's 1980 rate was the highest since 1944.

The railroads included by the ICC in its list of 37 were those with annual revenues of at least $500 million. Yesterday's report showed that 5 railroads were earning a rate of return between 9 and 12 percent, 16 railroads had a return between 5 and 9 percent, 7 were below 5 percent, and 6 were at zero.

In another action, the ICC proposed a major reduction in reporting requirements for the nation's railroads and trucking companies.

If the proposals are adopted by the agency following a public comment period, railroads would no longer have to file 21 categories of information in their annual reports to the agency, and trucking companies would be freed of filing requirements of 15 categories.

Among the items to be eliminated are officers' compensation, financial transactions involving affiliated companies and corporate disclosure information.

In a final action taken, the agency reduced some accounting and reporting burdens on interstate trucking and bus lines by raising from $200 to $500 the minimum amount they have to report to the ICC as a capital expenditure on their balance sheets.