Three grown men in jackets and ties are standing in a small laboratory watching the end of a wire.

Suddenly, in the center of the cross section at the end of the rubber-coated wire, a light flashes on and off. The three men lean back and smile.

These men are paid to watch a light at the end of a wire go on and off. Well, not exactly a wire, but glass fibers running through the center of some other glass and a rubber jacket. It looks like a wire and, more remakably, it does what only wires used to be able to do.

These men are working on a significant technological breakthrough, the transmission of voices by laser beams over glass fibers. Widespread use of laser and glass technology could result in huge cuts in cost and simultaneous improvements in service, and these technicians already have made the most important breakthroughs.

They work for the American Telephone & Telegraph Co., the telephone company, at Bell Labs, a huge research facility in the rolling hills of Western New Jersey.

And in many ways, Bell Labs and its half-step into the future is fast becoming AT&T's ace in the hole. Faced with a new era of deregulation, the phone company is finding more and more competition in areas that used to be its exclusive jurisdiction. Some of the toughest and most aggressive competition in supplying communication services is now coming from other giant conglomerates such as International Telephone & Telegraph Co. and IBM.

And AT&T, which for most of its history has been in virtual control of its own destiny, now has to worry about giving customers what they want, not what Ma Bell thinks is best for them.

So Bell Labs, which is the research arm of the telephone system, is changing gears and, with the help of a billion-dollar budget, is learning how to cope with the demands of the public. And because of its extensive research into alternative forms of communication, Bell Labs gives AT&T just enough of an advantage to keep the blood pressures of its executives-nervous about entering the strang world of marketing and competition - down to manageable levels.

The lightwave communications research program is just one example of how AT&T is staying on the cutting edge. But it is an important example because it has tremendous ramifications.

"The use of fibers has only become practical in the 1970s," said Ira Jacobs, a director of the Bell Labs Transmission Systems Division. "But the fact that fibers could be used to transmit light has been known for a long time, about 50 years. The critical issue has been making the fiber so pure that there is nothing to stop the light."

Using lasers and fibers makes possible considerable savings in size and economics. A single pair of hair-sized fibers, or "lightguides," can carry 672 simultaneous conversations. And certain qualities of the fiber system make it particularly attractive for such things as picturephone visual service or high-speed data transmission.

And there are other advantages. Because light-wave communication is immune to electromagnetic inteference, it can be used near power stations and other environments that frequently cause problems for traditional wires.

For two years, AT&T has been testing a light-wave installation in Chicago in a 1 1/2-mile link under downtown streets between two Illinois Bell Telephone Co. switching offices, and between one of those offices at a downtown Chicago office building.

"The system performed even better than expected," said Jacobs, "providing dependable service with virtually no down time and surpassing even the most stringent performance objectives. In fact, not a single customer phone call has been interrupted or adversely affected as a result of trouble with the light-wave components."

That Chicago system requires a cable only 1/2 inch in diameter - a fraction of the size of traditional cables used to interconnect central offices in cities.

But light-wave represents only a small part of the research activity at Bell Labs, where several other areas of research are being pursued in an attempt to shift some emphasis of the 18,000-person Bell Labs to the long-ignored area of marketing.

Several people are working in the area of acoustic research, or speech recognition. They are trying to computerize what happens when people talk, i.e., how speech is made. Then, by using speech patterns, the company can offer a whole host of products that will respond only to the voice commands of the right person.

Although they are getting better at it, they are still not as effective as, say, fingerprints, for identification. But, as one Bell researcher said, "Imagine the possibilities." There are concurrent experiments in making computers "talk" in human-sounding voices. Such research could lead to the use of a push-button telephone to gain access to a computer that could "speak" its answer to a question expressed in numbers - thus eliminating the need for a printout device or screen.

In the area of microelectronics, Bell is working feverishly to reduce the bulk of the equipment it needs to run its huge network. A new semiconductor memory device developed last year contains 65,536 storage cells on a single silicon chip - a 64-fold increase in memory capacity since 1973.

And microelectric devices are now found on almost all new equipment used by the Bell System.

Bell engineers also nearly have doubled the speed at which electrons can move through semiconductor crystals, a discovery that could reduce sharply the amount of power necessary to run such products as stereo systems, computers and, of course, complex telephone switching systems.

But the bottom line for Bell Labs is changing and, as its chairman, Willian O. Baker said in a recent interview, "Our security blanket has always been thin."

In fact, as Baker pointed out, "Performance is always based on competition." But the lack of competition over the years for Bell Labs did prove somewhat costly in terms of marketing expertise. "It we weren't discovering it, we wouldn't use it," he said.

"We are bringing in marketing people," said Baker, naming a few already hired from IBM. "But we already have a yound and energetic staff."

And the new thrust of Bell's efforts are towards "custom-tailored" communications systems for customers who were being wooed by competitors. Now Bell can respond to a customer who informs it that another firm has proposed an entirely new office system designed to save time and money.

"We can be competitive," Baker said. "And we will be. We are going to die in the ditch." CAPTION: Picture, Hair-thin fibers are used to carry voice, video and data by laser in Bell Lab's test in Chicago of its light-wave communications system. Bell Labs