Hi Tom,

in short: up to a point

the add ingredients extension allows you to setup custom groups (for lack of a better word)
i.e you could say “pizza a” allows to add dessert a or b or c for example (free or plus some additional price per topping – up to you)
and various other options (if you go to http://www.wp-pizza.com/twentytwelve/ and select “Make Your Own Pizza” you can get an idea. happy to expand/explain more if required.

furthermore, i am in the middle of testing an updated version of this extension that allows adding toppings for half/quarter pizzas (or whatever meal you choose) as well as pre-selecting or excluding toppings on a per menu item (meal) basis and a few other bits and bobs. this hopefully will be ready in a week or so ( I will make an example available in the 2012 theme somewhere)

having said that , at the moment there’s no option to say “order 3 for x £” etc and add toppings to each
(something i need to look into in conjunctions with so-called coupons i suspect)

although you could call your menu item “3 pizzas” of course for a certain price you set depending on size, I suspect it would get messy or at least not very obvious to whoever fulfills that order….

happy to provide more detail if required