With the magic knack trait u get 2 caster level so
3wizard/2rogue can get 3rd level spells on scrolls, add them to spell book and memorize it? Or just heightens the rounds per level and in case of fireball oraomething do 2d6punk more damage?
It's the second option, Magical Knack causes all of your wizard spells to function as if 2 levels higher to a maximum of your Character level (In this case 5)
EDIT: Also, I would change the name of the thread because you haven't actually referenced anything directly related to the Arcane Trickster Prestige Class, unless of course there are more questions
Sincerely A Ninja
The two bonus caster levels only affect the level-variables of spells you can cast by virtue of your class levels in a spellcasting class; they aren't extra CLASS levels, so you don't get more/higher-level spells. Just get better with the spells you can already cast.