score:5
I looked at the VRR and Rheinbahn sites and it seems your best option is to buy a day ticket (Tagesticket) every day. It costs 6.80 Euros and allows unlimited travel within Duesseldorf.
Single fares are 2.70 Euros, so if you do 3 or more rides, you're better off using the day ticket.
There is no 3-day pass available, just day tickets and weekly/monthly tickets.
The farelevel from the airport to the center is A2, while the day ticket is available for levels A,B,C,D and E. It is not totally clear on the web sites, whether the day ticket covers the airport.
This page covers both A1 and A2 and lists a single price for the day ticket: β¬6.80, so I assume you are fine with a day ticket level A for the airport.
Upvote:2
I live in D'dorf and what @Peter Hahndorf says is the right option. If you plan to take bus or train more than 3 times, buy a tagesticket every day.
The Tagesticket is valid in all farelevels, so you will not need to pay more for going to the airport. Here just to say there are 2 airports serving DΓΌsseldorf, "Dusseldorf-airport" and Weeze (ryanair). Weeze is about 100km. from D'dorf, but the tagesticket is valid for both. You could also visit Cologne without paying more!