Selecting columns by name is easy enough:
dt = Current Data Table();
a = 2;
b = 7;
c = 9;
Column( dt, Char( a ) ) << Set Selected;
Column( dt, Char( b ) ) << Set Selected;
Column( dt, Char( c ) ) << Set Selected;
But I'm not sure you need to select them to create the overlay plot you are looking for. You can just do this:
Overlay Plot( X( :Name( "1" ) ), Y( Column( dt, Char(a) ), Column( dt, Char(b) ),
Column( dt, Char(c) ) ) );
If you have a variable number of Y variables, you can construct a list of them and set that list as the Y variables for overlay plot:
colList = {};
colList = Insert( colList, Column( dt, Char( a ) ) );
colList = Insert( colList, Column( dt, Char( b ) ) );
colList = Insert( colList, Column( dt, Char( c ) ) );
Overlay Plot( X( :Name( "1" ) ), Y( EvalList(colList) ) );
HTH,
Eric