Declare a List and populate with values using one code statement

var list = new List<IMyCustomType>{ 
    new MyCustomTypeOne(), 
    new MyCustomTypeTwo(), 
    new MyCustomTypeThree() 
};

Edit: Asker changed "one line" to "one statement", and this looks nicer.

var list = new List<IMyCustomType>
{
   new MyCustomTypeOne(),
   new MyCustomTypeTwo(),
   new MyCustomTypeThree()
};

Not quite sure why you want it in one line?

use the collection initialiser

var list = new List<IMyCustomType>
{
   new MyCustomTypeOne(){Properties should be given here},
   new MyCustomTypeTwo(){Properties should be given here},
   new MyCustomTypeThree(){Properties should be given here},
}