you need to put a formula into the third column
which can then be dragged to replicate it throughout the column
put the cursor in a 3rd column cell and click on the "insert function" icon
select the "text" category of functions
then select the "concatenate" function
build up the formula as required, an example can be found here
http://www.techonthenet.com/excel/formulas/concat.htm
you should now realise that a suitable function could be derived to generate column 3 from columm 1 alone
all that remains now is to drag the formula throught the rest of the column (click bottom right corner of the cell)
Edit
took too long to type that and was beaten by Raistlin