Three little boxes of flashing red lights inside a bank in Bethesda are apparently the solution to one of the major problems of the Three Mile Island nuclear power plant accident last March.

The lights indicate separate telephone lines that go from the Nuclear Regulatory Commission emergency operations center in the bowels of the Maryland Bank building to each of the nation's nuclear reactor sites and to all 14 radioactive fuel handling locations.

The problem it is hoped they will solve is busy signals, the ones that greeted the NRC when when it was trying to call the people running Three Mile Island as radiation began billowing out over Pennsylvania.

With the new hotlines, NRC engineers on duty 24 hours a day can be in immediate communication with control room operators anywhere in the country. The system cost $500,000 and will cost $1.2 million a year to maintain when another set of phones is installed. The second set will link the NRC's radiation specialists with their counterparts at each reactor so that any release of radiation can be monitored closely in Washington.

"This means we can deal with situations we haven't anticipated and can go right to the source for information," said NRC Commissioner Victor Gilinsky, who prodded the Project into operation June 1, a month ahead of schedule.

NRC officials complained during Three Mile Island that their information in the early hours of the accident, when crucial decisions were being made, was scanty, secondhand and later. The phone system, Gilinsky said after a recent tour of the facilities, also guarantees the direct and early involvement of the NRC in any future crisis.

All conversations on the hotlines are taped on a 20-channel receiver that looms over the emergency center near charts, blackboards and diagrams of reactor wiring. A conference room with speakerphones, slide projectors, closed-circuit television and a computer nearby can put NRC officials in touch with all available information, according to Vic Stello, director of the NRC's office of inspection and enforcement.

The nation's nuclear plants have been instructed to call in on the hotlines with reports of any major radiation release, any reactor damage likely to run over $200,000 and any radiation-associated injuries. Although the duty officer's logbook is already two inches thick from the first month of operation, only one call on a reactor problem has been received. That one, reported June 7, concerned a brief leak within the containment of the plan at Rancho Seco in California.

The other notations record tests, routine check-ins and wrong numbers. One caller wanted to know if the NRC gave reactor operators psychological tests (it does not) and another wanted to know why the emergency center hadn't called him lately (the test scheduled had been trimmed).

Some incidents that ought to have come immediately to the center's notice this past month were first routed, out of operator habit, through the NRC's regional office, said Stello's deputy, Dudley Thompson. "That will change as things become more of a routine," he said.