1) only col1
to use an index, you need data to be ordered in this index (else the index is not useful). as far as the index is concerned, if you set col1, data will be ordered in col2, if you set col1 and col2, col3 will be ordered, but if you only set col1, col3 will NOT be ordered (was I clear ???)

2) consider that none is better as far as execution time is concerned ... one will be bigger that the other, anyway you shouldn't choose according to the performances, but according to what your needs will be, and to what your queries will be like ...

3) you mean a kind of performance index that would be comparable between queries ??