It really isn't that difficult:(M Missinary, C Cannibal, B Boat)
1. MMMCCCB (nothing)
2. MMCC MCB
3. MMMCCB C
4. MMM CCCB
5. MMMCB CC
6. MC MMCCB
7. MMCCB MC
8. CC MMMCB
Now just do the inverse of steps 4 to 1 to end up with everyone having crossed the river.

If you like puzzles like this, I have another one for you:

A ship and it's (or is it her?) captain together are 56 years old. Now, the captain is twice as old as the ship was, when the captain was as old as the ship is now. Now , how old are ship and captain.

Nice holidays,

Auke