Three of the nation's most profitable railroad companies today reported first-quarter results that reflected the exposure of each to bitter winter weather during January and February.

Union Pacific Corp., whose Mid-west-Pacific Coast operations were not affected much by the Northeast miseries, reported a 35 per cent jump in profits to a first-quarter record high of $47.6 million ($1.01 a share) compared with $35.3 million (76 cents) in the 1976 period.

Washington-based Southern Railway, which experienced a "stronger-than-anticipated-recovery late in the quarter after cold weather had exacted its toll, also posted record first-quarter profits of $27.6 million ($1.82a share) compared with $22.1 million ($1.45) a year earlier, a more modest, gain of 25 per cent.

Norfolk and Western Railway Co., hit much harder by bad weather in January and February because its lines run deep into the Northeastern states, reported a 36 per cent decline in first-quarter profits to $17.7 million (57 cents a share) from $27.6 million (88 cents) in the 1976 period. "But we were in the black," N and W president John P. Fishwick said in an interview," . . .and I think that's probably the only railroad in the East" with such a performance.

Of the other major northeast carriers, Chessie System has posted a first-quarter loss because of the weather and Consolidated Rail Corp. is expected to report a loss much bigger than projected in government plans for the new railroad.

Seaboard Coast Line Industries, meantime, reported that first-quarter profits were up slightly to $1.38 a share from $1.37, on net income of $20 million for each period. Revenuse rose to $410.7 million from $371.1 million and the company named president Prime F. Osborn as chief executive to succeed W. Thomas Rice, who continues as chairman.

Union Pacific chairman Frank E. Barnett said his firm's revenues increased 22 per cent in the quarter to $588 million. Rail profits were up 34 per cent and earnings of the company's Champlin Petroleum unit were up 19 per cent, Barnett reported.

Rail traffic for nearly all commodities increased but particularly for intermodal trailers, automobiles, mail, soda ash and coal. UP's chairman urged Congress to amend President Carter's energy package to increase emphasis on stimulating development of oil, natural gas, coal and uranium. "Conservation by itself" will not meet the President's goals, he asserted.