They should go in the order of Hughes, Hussey, Ponting
Can't do it all at once though
Well, of those three, Hughes will DEFINITELY be gone after this test. With Watson coming back, Warner's century in this match consolidates his position as the other opener for the Boxing Day test. As someone posted yesterday, although the opening partnership was building steadily, in reality Warner and Hughes were competing against each other for the other opening spot alongside Watson.
Regarding Hussey and Ponting, I cannot see either of them dropped by the selectors until the end of the test series vs India at the earliest. If neither of them perform during that series, at 36 and 37 yrs old respectively, both of them will be forced to retire. However, Ponting is under a lot more pressure than Hussey, as Hussey has been Australia's best batsman for the past 12 months (dating back to the Ashes last year, not to forget his performance vs Sri Lanka), whereas Ponting's Mark Taylor/Hussey/Hayden streak of 30+ innings without a century is definitely under the microscope. Excluding the dodgy 205 he got vs Pakistan, his last legitimate century was back in July 2009 at Cardiff