IT'S HARD TO believe that the United States was unaware that there were "3,000 Soviet combat troops" in Cuba. The State Department's story is that our intelligence experts were caught by surprise. It was only after they "reevaluated" their reports of the past five years that they concluded the Russians were there.

I managed to get one of the CIA'S folders containing intelligence information from our man in Havana, a certain Miguel X, which explains why the United States was never sure that there were Soviet troops in Castro's Disneyland.

The first memo was dated Aug. 12, 1974.

"Three thousand Soviet tourists disembarked from the passenger ship 'Lenin' in Havana at 2 a.m. this morning. They were dressed in white wash 'n' wear seersucker suits, wore Panama hats and were carrying East German cameras and Ivanov submachine guns. I inquired of their tour leader, Lt. Gen. Vladimir Sokolov, what they planned to do in Cuba and he replied, 'We are from Kiev-Cuban Friendship Society to see beautiful country, sightsee and do cha cha with Cuban girls. We get five-year tour of country for package price of 995 rubles with breakfast included.'

"Do you want me to follow up? -- M.X."

The reply to Miguel X was also in the folder and said, "It sounds like just another Intourist junket to us. If you can get anything you can pin down, let us know. Otherwise, due to budget restrictions, prefer you forget whole thing."

The next memo from Miguel X was dated Dec. 3, 1976.

"Sorry to bother you, but was passing by Manzanillo and accidentally ran into Russian tourists. They have set up mobile homes with large saucershaped electronic equipment on top. Also saw 50 antiaircraft guns, 200 tanks and heard artillery firing off in the distance. Ran into Gen. Sokolov in a bar in town and asked him how he was getting along. He bought me a vodka and said, "Tour is going along fantastic. There is so much to see in Cuba we don't ever want to leave.'

"I asked what the saucer-like discs were doing on the mobile homes and he said, 'Is for television. Reception in this part of country is very lousy.' Then I asked him about the artillery firing. 'Is nothing but our people hunting wild boar. Look, we have license signed by Castro himself. Each tourist is entitled to one boar.'

"I can't put my finger on it but I think I was given a snow job. -- M.X."

The answer from his superiors in Washington read: "Thank you, Miguel, for your report. We wish you wouldn't bother us with your suspicions that you were being lied to. It's obvious the Soviets have moved into mobile homes to save on hotel bills. The TV story makes sense to us. As for the antiaircraft guns, they were probably left over from the Bay of Pigs. Because of new cut in budget, would prefer you mail your urgent reports via Mexico and not use straight cable."

The last message from Miguel X was dated May 1, 1979.

"The Soviet tourists held their Fourth Annual May Day Parade in the city of Batabano. Gen. Sokolov, who has been promoted to marshal, was in the reviewing stand with Fidel Castro. I managed to talk to him after the parade. He said he was having the best visit and the only thing he missed was black bread and borsch. He was thinking of opening a Russian-type restaurant which would cater to tourists like himself who were getting tired of Cuban food.

"The tourists put on quite a show for us. They divided themselves between the Red and Blue tour groups. The Blue group tried to capture the beach and the Red group repulsed them with mortars and machine guns. Of course they were using blanks. I don't know why I still think there is more to all this than meets the eye."

The final reply from Washington said: "Please stop wasting your time lolling around the beach with some bathers who just seem to be having a good time and concentrate on any military activity in the country."