If col3 is null, then MyCol will be null; so you should do something like shown below which uses COALESCE function to take care of nulls.
This considers the possibility that any of the 3 columns can have null values. If that is not the case, the expression can be simpler.
This is also extensible. If you have a fourth or fifth column, you can follow the same pattern as the one used for col3 and col4.
SELECT col1, LTRIM
+ COALESCE(' '+NULLIF(col3,''),'')
+ COALESCE(' '+NULLIF(col4,''),'')
) AS MyCol