A federal narcotics investigation at the Pentagon has led to arrest of three civilian Defense Department employes and an alleged drug supplier on cocaine distribution charges, authorities said yesterday.

Three of the four alleged cocaine dealers have pleaded guilty to narcotics offenses as a result of the probe carried out by agents of the Drug Enforcement Administration. Authorities said the investigation has ended.

Robert D. Cooke, 24, a Pentagon loading dock employe, pleaded guilty in District Court in Alexandria yesterday to a single count of using a Pentagon telephone to arrange a $1,050 cocaine sale to an undercover federal narcotics agent. He faces a possible maximum sentence of four years' imprisonment and a $30,000 fine.

Earlier, Carleton C. Maxwell, a Pentagon driver, pleaded guilty to a similar charge, officials said. He was sentenced to three years' probation and ordered to perform 500 hours of community work. Ricardo W. Johnson, another Pentagon driver, was also indicted in Alexandria on cocaine trafficking charges. His case has been transferred to the District of Columbia for prosecution.

Edward Lee Harper, an alleged narcotics supplier, pleaded guilty last month to conspiring to distribute cocaine, officials said. He was ordered to serve five years' probation and perform 1,000 hours of community work.