Airberlin has a very interesting Topbonus partner promotion. You can earn 2000 bonus points for two partner transactions and 3000 bonus points for three partner transactions. This offer is valid for partner transactions between June 15 and August 31, but you need to register by July 31 and you can do it here.
Airberlin’s Topbonus has number of partners and most of them are eligible for this bonus. Seems that you don’t need to have three different partners but merely three different partner transactions qualify: