Insert all column dates into one separate column

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SQL-Server

SQL-Server-2008

Hi ,

I have a doubt in SQL server.

I have 3 date columns , my columns is look like this...

ID       COL1        COL2        COL3         
1        JAN-2011    NULL        FEB-2013
2        NULL        NULL        NULL
3        NULL        DEC-2012    NULL

NOW, i need a column like

ID       COL1        COL2        COL3          col4
1        JAN-2011    NULL        FEB-2013     jan-2011
1        JAN-2011    NULL        FEB-2013     FEB-2013
2        NULL        NULL        NULL         null
3        NULL        DEC-2012    NULL         dec-2012 

How to achieve my desired output.
Can any one help me out of this....