I have a list in C#:
var list = new List();
list.AddRange(GetGreenCars());
list.AddRange(GetBigCars());
list.AddRange(
Assuming your Get*Cars() return Lists of Car, another option could be:
var list = new List();
GetGreenCars().ForEach(c => { if (!list.Contains(c)) list.Add(c); });
GetBigCars().ForEach(c => { if (!list.Contains(c)) list.Add(c); });
GetSmallCars().ForEach(c => { if (!list.Contains(c)) list.Add(c); });