Nippon Yusen Kaisha, Japan's leading ocean carrier, says it is negotiating to take over three ships operated by Showa Line Ltd., another Japanese operator.

Showa has three liner services to the United States, all operating in a space-chartering arrangement with NYK Line. Under such arrangements, the lines share space on ships but market that space in competition with each other.That arrangement will continue until June 1988 in each of the three services, said Akihiro Takei, managing director and North American administrator at NYK's U.S. headquarters in Secaucus, N.J.

NYK will maintain all its present services, Mr. Takei said in a statement.

Showa Line Ltd., one of Japan's six top shipping companies, said last week it will end its liner services between Asia and North America. Showa officials cited the prolonged slump in its liner business. NYK was the only Japanese line that reported a profit among the six lines for the fiscal year 1986, ending March 31 last year.

A consolidation of Japan's six big carriers is widely expected, possibly by the end of the year. The Japanese lines have reported huge losses due to the sharper value of the yen and the shift in production to other Asian countries.

Meanwhile, freight rates have been depressed and additional capacity has been added on the trans-Pacific trade route, the world's busiest.

Showa officials in Tokyo have said that dropping the liner services will help the company return to profitability. They said the company will emphasize the tramp and tanker divisions.