How to combine two flat lists into one nested object

Question

I have three lists which contains three same properties in each collection. I want to combine a result into one collection. Ex classes structure is as below

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Answer 1

You can use inheritance.

public class ResultCollection : Collection1
{
    List Collection2s { get; set; }
    List Collection3s { get; set; }
}

and then

var result = new ResultCollection {
    PropId1 = Collection1.PropId1,
    PropId2 = Collection1.PropId2,
    ...
    Collection2s = Collection2,
    Collection3s = Collection3
}

An automapper can be helpful here.
https://docs.automapper.org/en/stable/