Ask him about the secret of life, the universe and everything. If he says 42 then he will be of no use to me :) 

Desire is not suitable here in place of wish here.

(Imagine you found a lamp with a genie inside, and you could fulfill any three desires  - or - and you were granted three wishes, what would they be?)

if you could fulfill/satisfy only one desire, what would it/that be?

<em>when you're dealing with a genie in a lamp situation, it's usually "three wishes", a bit simpler that way</em>