NEW DELHI India on Monday successfully conducted three underground nuclear tests, defying international opinion, Hindu Nationalist Prime Minister Atal Behari Vajpayee announced here. Vajpayee told a hurriedly convened press conference at his official residence that the tests were conducted at 3:45 p.m. (1015 GMT) at a nuclear test ground in Pokhran in the northern desert state of Rajasthan. Vajpayee, who assumed office in March at the head of a minority government, said the tests were conducted with a fission device, a low yield device and a thermonuclear device.