The higher you go, the harder it gets.
Personally I would count up the ground floor windows, and for arguements sake, charge 75p per window, 1st floor @ £1 per window, 2nd floor @ £1.75 per window and 3rd floor at least £2.50 per window.
Those charge rates would be miimum charges for me, but it really depends on you and the area in which you work.
If these are posh flats then load up the price more than you would normally charge.
Also check to see how long it takes to get paid, that should also make a difference as to how much you should charge.
I presume the residents of these flats have a service charge payment the make to the landlord?
If by 3 storey you mean, ground, 1st, 2nd and 3rd floor, then that is quite a lot of high work, don't be afraid to put in a big price, and don't worry about being undercut!!
On work like that you need good prices to make it worthwhile
regards,
Ian