If OP is accurate when he says he doesn't know "the first three" words, then it is only 20483 possibilities, which is solvable in hours to days, rather than years.
On the worst CPU it should take from 6 hours to half a day tops, which is slightly faster on a better CPU. Also with a decent GPU it could become so much faster that the required time would come down to about an hour or less. We are talking about only 8.5 billion permutations after all.
Successful launch of btcrecover. With three words missing (bigtypos 3), it uses astronomical amounts of ram and the computer freezes.
I have read the instructions for btcrevocer and I wonder if it is possible to set using regex, for example words starting with the given letter "a% b% c%"?