Here is a hackish solution for Excel if you can't program:
1. I'm assuming a kw list in column a, col B blank
2. Insert a blank row above the list (row 1 is blank)
3. In B2 put the formula =FIND(" ",$A2)
4. in C2 put =FIND(" ",$A2,$B2+1)
5. In D2 put =FIND(" ",$A2,$C2+1)
6. select b2,c2,d2 and drag the the bottom of the kw list (or double click on the bottom right corner)
7. select row 1
8. choose Data-> FIlter-> Auto Filter
9. on the col c dropdown choose custom-> does not equal -> #VALUE!
10. on col d dropdown choose #VALUE!
11. you are left with a list of all the 3 word kws
(there is probably aneasier way in excel, cannot think of it right now)