There is one other way though,
I could connect one monitor to DVI, the second with an adapter to the HDMI port and the third one with an adapter to the apple-mini-displayport. Will this cause any problems and/or difference in the display-quality?
EDIT: I read your post again ;p
When you use one DP-connected monitor with any other two types, HDMi/DVI/VGA, you will get tearing on the DP-connected display.
Then perhaps I should get two adapters to convert two DVI monitors to the two apple-mini-displayports. Then I can connect the third monitor the DVI or HDMI port. Is that an option? I was wondering how you did it with your XFX HD 6870 1 GB GDDR5, because that card also has two DVI, one HDMI and two apple-mini-displayports ( http://content.hwigroup.net/images/products_larges/115794/4/xfx-radeon-hd-6870-black-edition-v2-1gb.jpg )
Could you tell me more about that?