Hi Rusana
First of all you should understand the concept and use of costing variant at different object level cost calculation as well as to calculate standard cost/plan cost.
1. PPC1 is used when we calculate standard cost in CK11N/CK40N to update cost at costing view 2 in material master
2. PREM is used when we calculate preliminary cost at Cost Collector level
3. PPP3 is used to calculate actual cost at Cost collector that means when you post GI/Activity through Back Flash system calculate cost for actual posting based on PPP3
Basically when we calculate standard cost at CK11N/CK40N no actual cost is required to be calculate..so only PPC1 is used to calculate Plan/Standard cost
But when we are calculation object level costing always a preliminary cost is calculated..you may call it estimated cost of plan production qty and Actual cost for actual consumption of goods & activity.So 2 costing variant are used at every object level cost calculation.
In your case..PREM for preliminary cost and PPP3 for Actual cost at cost collector level
I think this will clarify your question
Kamal