My customer wants a three tiered cake and all of it iced with a whipped icing. I was wondering if it is ok to do a tier cake with this type of icing. I have only done with buttercream or fondant.

I suggest you google this specific phrase:  pastry pride 3 tier cake   

You will see images and lots of different threads about what can be done with stabilized whipped icing.  This is VERY different from buttercream or fondant.  There are some limitations. Fondant can only be put on at the last possible second, or it will become gooey and disgusting.  A whipped icing cake may need to be refrigerated.

Look at the cakes done by Crosbycakes, a CC member, http://www.cakecentral.com/u/939332/crosbycakes/gallery  

This is one of her Pastry Pride cakes:

http://www.cakecentral.com/gallery/i/3200751/this-cake-is-iced-with-pastry-pride-the-flowers-are-fondant

Bettercreme is another brand of whipped icing.