Australian dollars are not used in Europe inthismoment. It is always better to post in the local currency. It is much easier for your reader to understand. However, as you are a new poster I'll do the conversion for you.
You have a budget of approx. 5700-6500 Euros. You have already pre-paid accomodation at a cost of approx. 3600 Euros. This leaves you with approx. 2100-2900 Euros for food and other costs.
Generally speaking, the accepted minimum backpacker budget (hostels, supermarket food) for W. Europe is around 50E per day including accomodation but not including transportation. If your total time is 90 days, that works out to 4500E. So your total budget seems doable but there are a couple of questions you need to supply answers to.
What accommodation have you booked? Hostels? 3Star hotels? Hostels will probably be between 10-30E per night depending on location. So an average of 20E would tell you that for 90 nights it should be somewhere around 1800E. You say you have pre-paid double that. So it sounds like you have booked hotels. If you are using hotels, you are probably not planning to buy some food at a supermarket and cook it in the communal hostel kitchen. Restaurants will of course cost more.
So you see, you need to provide more info. How many days specifically are you planning to spend. What kind of accommodation are you planning to use. Restaurants or supermarket food. How are you planning to get around and where is the cost of that in your budget. Entrance fees and 1 beer a week or 4 rum and cokes a night.
The more relevant info you provide, the more relevant the response will be. Otherwise, you are making assumptions and those responding may make assumptions. You know what assumptions do , they make an ass out of u and me.