The empty Soyuz spacecraft that linked up yesterday with the Salyut-7 space station is believed by American space experts to be a three-man version of the two-man Soyuz the Soviet Union has flown for the last 10 years.

Called Soyuz-T (believed to be for Transport), the unmanned Soyuz docked early yesterday at one end of the two-port Salyut space station, which is orbiting the Earth at an altitude of about 220 miles. The entire docking operation was done on radio command from the Soviet Union.

All the Soviets have said about the new Soyuz is that it employs "new technology," but U.S. space experts believe that the interior of the Soyuz command craft has been redesigned so radically that it can now carry three cosmonauts instead of two.

U.S. experts believe the Soviets have repositioned the parachutes that bring Soyuz back to earth and may have installed fuel cells in the spacecraft to replace its batteries in a weight-saving maneuver. Both these moves would make more room inside the cabin of the command craft, which was originally built for three cosmonauts instead of the two it has been taking.

If the Soyuz-T is a three-man spacecraft, U.S. experts point out, the Soviets will probably continue to use the Soyuz as a transport for cosmonauts for years to come. There had been speculation that the Soviets were developing a large spacecraft like the space shuttle the U.S. plans to fly next year, but an upgrading of the workhorse Soyuz would tend to dim that speculation.

Using a three-man Soyuz, the Soviets could transport cosmonauts up to the Salyut space station far more efficiently than they do now with a two-man Soyuz. The Soviets have said they want to turn Salyut into a "permanent orbital station with interchangeable crews," a goal that is difficult with a two-man spacecraft.

The current flight of Soyuz-T is believed to be the fourth unmanned test of the new Soyuz configuration. The first test was believed to have been Cosmos 869 in December, 1977; the second may have been Cosmos 1001 in March, 1978, and the third was thought to be a 60-day flight of Cosmos 1074 last winter.