WASHINGTON, March 26--Britain, France, and the United States laid down differently worded conditions today for a summit conference with Soviet Premier Nikita Khrushchev.

President Eisenhower took a middle-ground approach, saying the United States would be prepared to go into a Big Four heads of government meeting on the understanding that "constructive proposals" had been prepared beforehand by a foreign ministers meeting.

President Charles de Gaulle of France agreed to a summit conference only after "genuine progress" had been made at a foreign ministers meeting.

Macmilian's note to the Kremlin on the other hand said the British would be glad to take part in a summit conference "as soon as developments in the foreign ministers meeting warrant."

Tibet Revolt Spreads

TAIPEI, Formosa, March 26--Rebels in Tibet have cut supply lines of the Red Chinese garrison, forcing it to rely on airdrops from China proper, Nationalist China Vice President Chen Cheng asserted today.

Simultaneously with his statement that the revolt was spreading, President Chiang Kai-shek offered Tibet independence if the Chinese Nationalists regain control of the China mainland.

Space Platform Proposed

WASHINGTON, March 26--Some of America's brightest scientists are looking forward confidently to a manned space platform which could move about and return to earth at will.

This report came almost casually today from Roy W. Johnson, director of the military space program.

The idea is to spring a thousand-ton platform into position by means of many small atomic explosions.