In Fable 1 you could cast nearly your entire spell book. The transition to Fable 2 saw a dumbed down/simplified version of spell casting. A lot of the spells had been removed(mostly the 'buff' or 'aura' spells which had to be recast) and you could only really cast 3-4 different spells at a time. I believe in Fable 3 you are only allowed to 'hold' 2 spells at a time and you can combine their effects for a third spell. Obviously I didnt buy Fable 3 and this was the main reason why.
I am sure there will be some sort of radial menu or way to hot swap your spells out, I will be a sad puppy otherwise.