NEW YORK - Three pals stumbled on a MetroCard machine in Penn Station that gave them free fares - and they milked it for $800,000 over nearly three years before the MTA caught on, authorities said yesterday.

The thieves - including a former student at the prestigious Wharton School of Business - were finally nabbed only after a routine agency audit uncovered their scheme, which came courtesy of a software glitch in the machine in the Manhattan transit hub, authorities said.

"The odds of [the suspects] stumbling on this were astronomical," MTA spokesman Jeremy Soffin said.

The scenario "would only happen if you used an active debit card but had insufficient money in your account and it was from a smaller, nonlocal bank."

The suspects would turn around and peddle thousands of Long Island Rail Road tickets and MetroCards to other riders, sources said.

Their customers included friends who wound up placing orders with them as to which tickets they needed, officials said.