The hosts could recapture the Ashes with victory at the WACA after dominant wins in the opening two tests in the five-match series in Brisbane and Adelaide.
Injuries have severely restricted Harris's test career and an ongoing knee problem is threatening to prevent Australia from naming the same team for a third test in a row.
"I'm hopeful that everyone will come up, I think there's the obvious one in Ryan-o, who bowled today, and we'll see how he pulls up," Clarke told reporters at the WACA.
"I'm confident he'll be fine."
Bowling all-rounder James Faulkner, who was 12th man for the first two tests, is another injury concern after leaving Australia's training session in the WACA nets when he was hit on the right hand by a ball.
"He's gone for a scan as we speak so I don't have the details but it looked like a nasty hit, hopefully there's no break there," Clarke added.
Faulkner, who bowls with his left hand, would be the most obvious replacement for Harris should the 34-year-old fail to prove his fitness for the test.
Australia also have Doug Bollinger and the uncapped Nathan Coulter-Nile on standby so still have the option of going with a four-pronged pace attack on what is expected to be a bouncy Perth wicket.
"I think one of those three will come in and replace Ryan if he's not fit but, as I say, I'm very confident that Ryan will be okay," Clarke said.
Despite the success Australia's pace bowlers, Mitchell Johnson in particular, have enjoyed so far in the series, Clarke said he would be unlikely to send England in to bat if he won the toss for the third straight test.
"I find it extremely hard to send the opposition in for a test match," he said.
"I'll see what the conditions are like tomorrow. I've always looked at batting first in a test match because you give yourself the opportunity to win the game.
"Unless the wicket is extremely green, I'd find it hard to send a team in."
Clarke scored a century in both the 381-run first test victory in Brisbane and the 218-run victory in Adelaide and said he thought the Perth wicket, for all its bounce, would offer plenty of runs as well.
"It's always hard to start, particularly in Brisbane and Perth, and bowlers can get on a roll," he said.
"But once you get a start, Perth, like Brisbane, is as good a place to bat as anywhere in the world because you've got consistent pace, consistent bounce and a quick outfield.
"The wicket looks as good a wicket as a I've seen in Perth for a long time. So I think we're going to see another tough test match that will go five days."
While conceding he was wrong in Adelaide, Clarke continued his tradition of predicting the England team.
Clarke thinks seamer Tim Bresnan will come into the side and England will pick a specialist batsman at number six but thought the tourists would not be too bothered by the conditions.
"It's going to be hot, they're going to be long days," he said.
"I think England are far too experienced a side to be bothered by the pace and bounce at the WACA ... or by the heat."
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