Dear student
 
If the object moves such that its trajectory is not confined within any two dimensional plane, and three variables are needed to describe its motion then the projectile motion will be a three dimensional projectile motion.The following conditions can make it necessary to deal with three dimension projectile motion:(1) Wind:The wind can act as a source of an external force experienced by the object. The object will be accelerated in the direction of force and its trajectory would change such that three dimensions are needed to account for its position in space.(2) Shape of object together with air resistance:At times, the shape of the object can also play a crucial role in accounting for the fact if the projectile will be a two dimensional projectile motion or a three dimensional one. However, the shape alone can bring that change and there must be air resistance to help along by acting a means for external torque. RegardsArun(askIITians forum expert)