I'm not massively worried, to be honest. Cummins is clearly sick and bowling 10km below his normal pace. Bird is bowling medium pace straight, which isn't wildly productive. Lyon and Hazelwood have had their moments, but they can't do it alone.
Not to take away from the English - you can only beat what's in front of you, and it's hard to begrudge Cook a good innings - but I don't think Australia need to click into panic mode by any stretch of the imagination. I reckon Khawaja may have run into the selection axe though. Even his decent innings in this series have been awful and stuttering.