The Reagan administration wants to increase by 100 the number of nuclear warheads it can drop on the Soviet Union by deploying an additional 50 Minuteman III ICBMs next year, placing them in silos now occupied by older Minuteman II missiles.

Each Minuteman III has three individually targetable warheads each with 175 kilotons of explosive power, a total of 525 kilotons. The Minuteman II has only one warhead, though its one megaton yield is almost twice as great as the total for the three Minuteman III warheads.

In a statement released Tuesday, the Joint Chiefs of Staff said the planned Minuteman III deployments would "partially offset" the phasing out of 52 aging Titan II ICBMs, which is scheduled to begin this year.

The Titans, largest and oldest of the U.S. ICBMs, carry an enormous nine megaton warhead. Their toxic liquid fuel, however, has made them too dangerous and too expensive to keep operational until 1986, when the larger, 10-warhead MX ICBM is to become available.

The idea of taking 50 spare Minuteman III missiles, which now are stockpiled for use in testing, and putting them in silos that hold older missiles has been around Capitol Hill for several years.

Many legislators oppose it on grounds the additional warheads would make no difference since the missiles would be just as vulnerable to a Soviet attack as the ones now in place.

The Air Force also fought the idea because it does not want to deplete the stock of 120 spare Minuteman IIIs. The extras, it argues, are needed to replace the 12 deployed missiles that each year are taken out of their silos and test fired from Vandenberg Air Force Base, Calif., to make certain the remaining systems will work.

Last year, however, a conservative group of legislators got Congress to approve $5 million to begin deploying the Minuteman III spares this year, and the Pentagon has decided to institute the program.

The Joint Chiefs also announced that when the first 40 MX missiles are deployed in Minuteman III silos, those Minuteman IIIs also will be put in Minuteman II silos.

These moves to increase the number of ICBM warheads, according to the Pentagon statement, "will arrest the downward trend in relative capability" between the Soviet Union's missile force and that of the United States.

It was also announced yesterday that three squadrons of B52D bombers, the oldest model of the venerable Strategic Air Command long-range bomber, would be retired.