East and West Coasts plus Tassie is a lot to see in 3 months but that said its a reasonable amount of time so you can see a fair bit. It all depends how much of the respective coasts you want to see and if you plan to fly between them or drive across the south of the country (and take the ferry to tassie) to connect all three. With a large country like Australia its always a balance between how much driving you want to do to tick off the bigger bucket list items vs exploring the smaller places off the highways where the real beauty often lies.