Wales duo Roberts and Phillips have hamstring and knee injuries respectively while England prop Corbisiero is troubled by a calf problem with a decision on their availability expected when the team is announced on Wednesday.
"We're hoping that they will be ready. The final decision won't be taken until Wednesday," Irvine said. "They are all making good progress. The medics are reasonably hopeful."
The visitors will take to the field for the series-deciding test without captain Sam Warburton, however, who was ruled out of the Sydney showdown with a "significant" hamstring tear.
The 24-year-old Welsh loose forward sustained the injury late in Saturday's 16-15 second Test defeat in Melbourne, as the Wallabies levelled up the series after the Lions had edged the opener in Brisbane.
Irvine admitted that the loss of "the outstanding" Warburton was a huge blow for the Lions, who are looking for their first series win since 1997.
"It's a serious blow because Sam started the tour with a knee injury but got better as the tour progressed," Irvine said. "In the first half, he was the outstanding player on the park. He's extremely disappointed that he's not available.
"On the positive side we're fortunate that it's one of the positions where we have tremendous reserve strength.
"We feel for Sam, he's been a super captain and role model to the lads. Hopefully we can go out and win the third Test for him."
Veteran Irish centre Brian O'Driscoll, if selected, will be in pole position to lead the Lions in the absence of Warburton and former tour skipper and lock Paul O'Connell, who broke his arm during the first Test in Brisbane.
Lions coach Warren Gatland will also have plenty of options to choose from for Warburton's replacement with Ireland's Sean O'Brien able to play both openside and blindside, while Justin Tipuric has been in brilliant form for Wales this year.
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