Which book is an absolute must for 3rd year?

1) Boards and Wards: A Review for the USMLE, Steps 2 and 3

2) First Aid for the USMLE Step 2

3) First Aid for the Wards: Insider Advice for the Clinical Years

save the step 2 books for fourth year and get yourself FA for the Wards. Read the relevant chapter the weekend before you start a new rotation to clue yourself into some of the basics.

Smurfette is right on! That's what I did and it scored some nice points. You at least have a clue what each rotation covers. And what isn't on the rotation.

Which book is an absolute must for 3rd year?

1) Boards and Wards: A Review for the USMLE, Steps 2 and 3

2) First Aid for the USMLE Step 2

3) First Aid for the Wards: Insider Advice for the Clinical Years
To do well on Shelf exams, none of the above is sufficient.

To have a clue what to expect on a given rotation (or attending/resident), the best thing to do (without the trouble of buying & reading) is to talk to someone from your class who just had the rotation. THAT information is priceless.