Using sed to cut a string starting with n spaces and ending with n spaces

How to use sed to cut text starting with x spaces and ending with y spaces?

For instance This is my string:

 kkk 111 fff      aaabbb 5d98 ccc         mmmppp 9369d

and I want to get this output:

 aaabbb 5d98 ccc

(the number of spaces is not known)

Thank you.