Can you explain what you mean with "Also, your statement is only true for the small number of rows..."?

I ran both your example and that from the question. On all queries the execution plan looks about the same, saying query 4 scans the table, while the others scan/seek the index with a dozen rows and 250+ rows in the table.

Since 3 columns (id,ch,na) are part of the index, no table scan should occure unless the forth column is addressed in the where clause.