Citicorp, parent of the nation's largest bank, yesterday reported its first-quarter earnings declined 3 percent, while Manufacturers Hanover Corp., whose subsidiary Manufacturers Hanover Trust is the fourth-largest, posted a 2 percent gain.

Two major telecommunications firms reported increased earnings yesterday. GTE Corp. said its first-quarter profits edged up 3.7 percent from a year earlier, while Nynex Corp. posted a 15.2 percent gain.

And in other earnings reports, PPG Industries Inc. said earnings per share for the first quarter of 1986 increased 2 percent to $1.17 compared with $1.15 for the same period last year. Caterpillar Tractor Co., benefiting from the dollar's recent decline, said it earned $111 million in the first quarter against a $70 million year-earlier loss.

The disappointing results for Citicorp and Manufacturers appeared to be mostly related to increases in loan loss provisions -- primarily for domestic loans by Manufacturers and in the consumer and multinational sectors for Citicorp -- and to a continuing buildup in reserves by both institutions.

*Citicorp, the parent of Citibank, earned $270 million ($1.87 a share) in the latest reporting period, down from $277 million ($2.02) in the first quarter of 1985.

Citicorp said only its institutional bank registered a decline in earnings, with income falling to $180 million from $205 million. Individual bank earnings increased to $97 million from $70 million, while investment bank earnings rose to $147 million from $100 million.

Provision for credit losses was hiked to $58 million in the quarter from $13 million in the year-ago period. Including other corporate costs and start-up activities, Citicorp charged a total of $109 million against earnings in the quarter.

Citicorp's total loan loss allowance amounted to $1.366 billion on March 31, up from $1 billion in 1985.

*Manufacturers Hanover Corp. earned $102.1 million ($2.14 a share) compared with $100.2 million ($2.81) a share a year ago.

Trading account profits soared to $19 million from $4.7 million a year ago, but foreign exchange revenue was down, as were gains from sale of securities.

Manufacturers Hanover Trust set aside $105 million as provision for possible loan losses, down from $108.7 million in the 1985 first quarter. Over the past year, its loan loss reserve has risen 24 percent to a total of $818.7 billion.

*GTE Corp., which is based in Stamford, Conn., said its first-quarter net income rose to $283 million from $273 million a year earlier. Per-share earnings were unchanged at $1.29 because GTE had 4 percent more shares outstanding than a year ago.

GTE's revenue climbed to $4 billion from $3.7 billion.

The company operates 15 domestic and three international telephone companies serving more than 12.4 million customers.

Its Sprint long-distance telephone operation, which pending regulatory approval is to be spun off into a joint venture with United Telecommunications Inc., continued to show losses in the period.

Revenue from GTE's communications services unit, which includes Sprint, rose 23 percent from 1985's first quarter to $395 million. But the unit showed a $52 million operating loss -- slightly less than the previous quarter's loss but nearly double that of the first quarter of 1985.

GTE blamed the latest losses on increased marketing costs, higher access charges and leased facility costs.

*Nynex Corp., which is based in New York, said its first-quarter profit climbed to $296.5 million ($1.47 a share) from $257.4 million ($1.275) a year ago. The per-share results reflect the company's 2-for-1 stock split on March 20.

Nynex's revenue rose to $2.7 billion from $2.5 billion.

The company provides telephone service for most of New York and New England, and it was one of the seven regional holding companies that were spun off from American Telephone & Telegraph Co. on Jan. 1, 1984.

*PPG Industries Inc., a diversified manufacturer based in Pittsburgh, said sales increased $101 million, or 10 percent, to $1.14 billion.

Net earnings for the first quarter of 1986 were down 14 percent to $69.2 million from $80.4 million the same period last year, which included an extraordinary gain $10 million(14 cents a share), the company said.

*For Caterpillar Tractor Co., which based in Peoria, Ill., profits were the company's best first-quarter showing in five years.

The leading maker of earth-moving and construction equipment said its first-quarter profits equaled $1.13 a share, and its sales rose 18 percent to $1.74 billion from $1.48 billion.

The last time Caterpillar posted higher net income for the first quarter was in 1981, when it earned $142 million.

Caterpillar, Illinois' largest manufacturing employer, lost $1 billion over a 3 1/4-year period before turning the corner with a $50 million profit in the second quarter of 1985.