NASA Friday postponed the crucial launch pad test-firing of Discovery's three main engines another three days when an attempt to fuel the space shuttle ran into problems with equipment not used since the Challenger disaster.

The postponement from Monday to Thursday was the third delay of the test in a week, and could put off the first post-Challenger shuttle flight - already delayed until late September - until October, officials said.Preparations for loading 500,000 gallons of liquid hydrogen and liquid oxygen into Discovery's fuel tank began Thursday night, but the launch team quickly had trouble with sensors and an oxygen pump.

Technicians resolved those problems but a leak in a hydrogen line and another balky oxygen pump finally forced a postponement early Friday.

The test will be postponed a minimum of 72 hours, with the launch team hoping to resume the first phase of the countdown Sunday night for the test-firing at 7:30 a.m. Thursday, said NASA spokesman George Diller.

It was the first time a shuttle had been fueled on launch pad 39B since the Challenger exploded shortly after liftoff Jan. 28, 1986, killing its crew of seven.

The first part of the countdown was to have ended this morning with a simulated problem and postponement just 10 seconds before a mock liftoff. The launch team then was to have emptied the fuel tank, started a new countdown and refilled the tank, targeting the test-firing for Monday.

The latest shuttle difficulty came a day after engineers came up with a solution for a major problem. They recommended that cutting a hole through Discovery's cargo bay would be the quickest and best way to reach and fix a small gas leak in an engine compartment without removing the vehicle from the pad.