If PPE consists of three major parts and each part has its significnat cost of the total cost. Is each part of PPE depreciated separately?

According to, International accounting standard (IAS-16) If an entity is following cost model of asset recognition than for assets which have significant cost parts in relation to its total cost, than such items must be depreciated seperately.

It depends on the framework, under US GAAP this matter is not treated and in practice not followed, under IFRS YES, If you have major parts that have a different usuful life than the principal asset you have to depreciate them separately according to their costs and usuful llifes. 

Yes, IAS16 does not prescribe the unit of measure for recognition – what constitutes an item of property, plant, and equipment. [IAS16.9]  that if the cost model is used, each part of an item of property, plant, and equipment with a cost that is significant in relation to the total cost of the item must be depreciated separately. [IAS16.43]

yes, and the big example is that of aeroplane

I support answer submitted by Mr. Zahid Chandio.

If an entity is following cost model of asset recognition than for assets which have significant cost parts in relation to its total cost

If that part is relevant to that particular PPE, and then the PPE cannot be in used without those parts, then it must be added as part of cost of PPE, and iwill be depreciated accordingly (retro or prospective).

Yes.