Originally Posted by
happiest_at_40000
if you really wanted to you can go airside at T3 and use the CX or QF lounges which are nice.
There have been several data points in the last couple of months of people being denied access to T3 when their onward flight is from T5, so it appears they are cracking down on this. I wouldn't recommend the OP plan on this unless he has lots of extra time, and even then it may not be worth it.