Lisa,
I think that at the start of TID, you take his current total daily dosage and divide by 3 to set the starting dose. However, there are a couple of people here who have been doing TID for quite some time, and they can tell you a lot more than I can. The way Bob responded to PZI, usually reaching nadir at +4 or +5, he probably would have made a good candidate for TID, but I wasn't here then to know to try it.
Carl