Thai forces fought for a third day to repulse Vietnamese troops who crossed the Thai-Cambodian border in an assault on a Cambodian resistance camp, and lightly armed guerrillas defending the cliff-top camp continued to hold out against their reinforced attackers, Thai military officials and western diplomats said today.

The Thai military reported this morning that Army troops supported by artillery and U.S.-supplied A37 Dragonfly aircraft had recaptured three hills seized Tuesday by about 1,000 intruding Vietnamese soldiers on the Thai side of Cambodia's northern border. Hundreds of Vietnamese were said to have been driven back across the border into Cambodia.

However, the Vietnamese counterattacked against Hill 361 on Thai soil behind the besieged Cambodian guerrilla base, and the results of the battle were not immediately clear, western diplomats said. Relief officials said skirmishing in the area was continuing.

Thai authorities said 14 Thai soldiers had been killed in the three days of fighting, three others were missing and 35 were wounded. In addition, they said, 15 Thai civilians were killed by Vietnamese shelling or gunfire.

Thai military officials said more than 70 Vietnamese troops had been killed in the cross-border operation.

The Thai military supreme commander, Gen. Arthit Kamlang-ek, accused the Vietnamese of deliberate aggression in attacking Thai military installations and defense lines with artillery and ground forces.

Vietnam dismissed the reported incursion as a "fabrication" and "slander" by Thailand. Radio Hanoi said Thailand's charge was intended only to "cover up its ever closer collusion with China in giving military aid to the Khmer reactionaries."

The reference was to three Cambodian resistance groups -- the Peking-backed communist Khmer Rouge, the anticommunist Khmer People's National Liberation Front and a faction loyal to Prince Norodom Sihanouk -- that have been battling the six-year-old Vietnamese occupation of Cambodia from bases along the Thai-Cambodian border.

In their most successful dry-season offensive along the border since invading Cambodia in December 1978, the Vietnamese have overrun all but one of about 20 guerrilla bases on the Cambodian side of the 450-mile frontier since November.

The one base holding out is a fortress-like camp called Green Hill -- now under siege -- that sits on a 530-yard-high escarpment a couple of miles across the border from the Thai village of Tatum in Surin Province. While the camp may be the most defensible of the guerrilla bases along the border, it belongs to the weakest of the three resistance factions: the Armee Nationale Sihanoukiste of former Cambodian monarch Prince Sihanouk.

"We're still holding," said Truong Mealy, a spokesman for the prince in Bangkok. He said resistance leaders who just visited the camp reported that the Vietnamese had not succeeded in cutting off access to it from Thailand and that "many attempts" to break forward defense lines so far had failed amid intense Vietnamese shelling and "quite heavy fighting." But he said only one guerrilla had been killed and 22 wounded in the fighting.

An official of the International Committee of the Red Cross said 31 wounded guerrillas had been evacuated for medical treatment inside Thailand since the Vietnamese attacked Tuesday, but that no figures for those killed were available. The camp is defended by about 5,000 guerrillas. Its civilian population of about 30,000 was evacuated during a Vietnamese attack in April 1984.

Western diplomats confirmed that the guerrillas were still holding the base today against elements of four Vietnamese regiments and one Cambodian regiment of the Hanoi-installed government in Phnom Penh.

Last night and this morning, the attackers nearly succeeded in cutting off defenders east of the camp, but the guerrillas managed to drive them back, the diplomats said. But they said that if the Vietnamese, who have received some reinforcements since the assault began, were further strengthened and were able to use Thai territory to flank the camp, it might not hold out much longer.