I haven't made this transition yet, in fact, I am just in the 2nd week of the transition from 5-4 with my 8 mo, but I agree with the general routine posted by Martina and Seona, a bottle in the morning, one mid afternoon and one before bed. As Martina said, the one before bed is probably the last one you will drop, my 3.5 still gets milk first thing in the morning and often before bed, it is part of her routine.
When I make the transition to 3 bottles- I intend on following the above schedule. Good luck!