As I understand it Darwin decommissions late this year, as for Melbourne and Newcastle, I'd assume they would both be decommissioned by, or around the time the last AWD commissions.
Earlier this year there was a media report (think it was DefenseNews?), that Poland was interested in the FFG's (at least the last two).
To back that up, there is an interview in the latest APDR with Christopher Pyne:
https://venturaapdr.partica.online/apdr/apdr-october-2017/flipbook/38/
If you have a read of page 38, Pyne says that he is off to Poland soon to discuss the possible sale.
If that happens, then Melbourne and Newcastle might well live on for a bit longer in Polish service, and who know Darwin might go as an active or spares ship too.
It's all happening!! FFG's to Poland, Classic Hornets to Canada!!
What's next??