As mentioned before you can do this with LEFT, MID and RIGHT
Cell A1 Contains: ABCDEFGHI
=CONCATENATE(LEFT(A1,3)," ",MID(A1,4,3)," ",RIGHT(A1,3))
Output ABC DEF GHI
To do this automatically though you would need to implement some cell watching in VBA i'd imagine to change the content to use the above formula based on whatever the cell was.