A spokesman said total casualties so far were three Americans wounded and four Communist soldiers killed.The news blackout lifted today was the most severe of * the 10-year-old war in Vietnam.It had given the Communist nations a field day for propaganda blasts against tlie United States and Aroused misgivings among some congressional leaders.The Communist nations charged that South Vietnamese . already, had Invaded Laos, and -the-Soviet-government-newspa-- -per Izvestia said on Tuesday American officers were commanding them. Communisl China said today the Allies were poised” .to invade Laos.And In Paris today the chief Hanoi negotiator, Xiian Thuy, and the .Viet Cong negotiator, Mme. Nguyen Thai Blnh, charged U.S,' and South Vietnamese troops actually were Inside Laos and that the Nixon administration was trying to cover un. I