capacitance between multiple plates

good morning;

i have a three plate 2D problem and i would like to calculate the capacitance between the relevant plates. over what surface do i need to integrate to get capacitance?

when i calculate capacitance do leave the 3rd plate floating if i compute the capacitance between plate 1 and 2?

i am using comsol 4.2

any help would be greatly appreciated
regards
gerhard