Here's how to find the position number of the third space in a string using Excel:

1. Use the
   Code:

   FIND

   function to find the position of the first space in the string:
   
   Code:

   =FIND(" ",A1)

2. Use the
   Code:

   FIND

   function again to find the position of the second space in the string, starting the search from the position of the first space:
   
   Code:

   =FIND(" ",A1,FIND(" ",A1)+1)

3. Use the
   Code:

   FIND

   function once more to find the position of the third space in the string, starting the search from the position of the second space:
   
   Code:

   =FIND(" ",A1,FIND(" ",A1,FIND(" ",A1)+1)+1)

For example, if you have the string "123 456 789 111 222", and you want to find the position number of the space between "789" and "111", you can use the third formula above and replace "A1" with the cell reference that contains the string. The formula will return the position number of the third space, which is 11 in this case.

Similarly, if you have the string "abcd efghi jklmnopqr stuvxy", and you want to find the position number of the space between "r" and "s", you can use the third formula above and replace "A1" with the cell reference that contains the string. The formula will return the position number of the third space, which is 17 in this case.