Although I´am famous and respected by everyone but they still won´t let me get passed the first 2 floors of the Imperial palace, is ther any way to get the key to the door up to the 3rd floor ?

Wel, I don't think that there's a way to get to the top floor but I may be wrong. However I think that you get to go up about 4 Floors(?) in the final thieves guild quest.

I could jsut use the console to set my acrobatics to 1000 and jump to the top, the problem would be getting down again

I doubt it, but try tcl from outside?

Console (~)
tcl

Make sure you have no one selected. Otherwise you wont float, this is the same as no clip.

The top has no collision....

You just fall straight through and are then either dead or stuck in the palace until you tcl again.