Undated (AP) _ Citicorp reported Tuesday that first quarter earnings tumbled 56 percent from a year ago, while three other large banking companies, NCNB Corp., Wells Fargo & Co. and Mellon Bank Corp., reported earnings gains.

In most cases, the Third World debt problem continued to play a significant role in the banks' results.

Citicorp, the largest banking company in the nation and based in New York, released quarterly earnings at the annual shareholders' meeting, announcing it earned $231 million, or 60 cents per share in the first three months of 1990. That compared with earnings of $529 million, or $1.52 a share, in the 1989 quarter.

NCNB, based in Charlotte, N.C., said earnings totaled $140.1 million, or $1.33 per share, 85 percent higher than last year's profit of $75.8 million, or 82 cents.

Wells Fargo, based in San Francisco, netted $159.8 million, or $3.04 per share, up 13 percent from earnings of $141.5 million, or $2.56 a share, a year ago.

Pittsburgh-based Mellon Bank earned $65 million, or $1.25 per share, compared with a profit of $53 million, or $1.10, in 1989.

Citicorp's quarterly income was hurt by flat revenue, $3.46 billion vs. $3.48 billion a year ago, as well as from increased writeoffs and a $58 million provision to cover shaky domestic real estate loans.

In addition, the 1989 results include a one-time net gain of $77 million from the sale of property in Tokyo, and $66 million in revenue from Brazilian cash payments that have since been suspended.

''The industry's exposure to heavily indebted countries is still seen as an important weakness and has necessitated further reserves,'' Chairman John S. Reed told the shareholders' meeting.

Citicorp's total credit loss provision rose $212 million to $570 million for the first quarter, reflecting higher write-offs in less developed countries and the $58 million in the real estate provision.

NCNB said it added $35 million to its allowance for loan and lease losses, bringing the total to $500 million. Average loans and leases rose 9 percent over a year ago to $34.4 billion.

The company said 1990 first quarter included the full benefit of the earnings of NCNB Texas, compared with NCNB's 20 percent ownership interest in the first quarter of 1989.

Wells Fargo said a lower loan-loss provision and higher net interest income offset an increase in non-interest expense during its first quarter.

During the first quarter, Wells Fargo completed the acquisition of two California banking companies, Valley National Bank of Glendale and Central Pacific Corp. of Bakersfield, with combined assets of $1.2 billion. The company said the acquisitions shouldn't impact 1990 earnings.

Mellon said its improved results reflected increases in revenue, including service fees and trading. The company also continued to reduce medium- and long-term loans to Third World countries. Its provision for possible credit losses was unchanged from the previous year's quarter at $35 million.