Is it possible to place three advertisements in the top where only two will change?
Example of the board which is going to be replaced:
http://www.politiekdebat.nl/forum
(http://img253.imageshack.us/img253/2852/pdbannerbf5.png)
No, but you could do this instead
In one "ad block", put the three you want to be shown
In another "ad block", put the two you want to remain the same, then another ad
In another "ad block", put the same two you want to remain, then change the third.. and so forth.