durkn
Presuming you have a cookshack, turn it up as high as it will go. I do quantities of them in my cs 250, up to 75 at a time. What I do is take them to 140 to 145 degrees internal temp. Then I take them out and hold them together in a container for an hour or so until slicing. During the resting time the internal temp will rise some, and you will have plenty of juice. After slicing pour the juice over the sliced meat. This always works well for me. Tender, moist meat, done but not dry. You can vary the doneness to your liking by varying the "take out" temp a little. You can also heat the juice before pouring over the sliced meat if it is a little too pink. I also trim almost all fat off before cooking. I've never had tough or dry tri tip by doing it this way.
On small quantaties - two or three roasts, after smoking to rare/medium rare, wrap them together in foil and if there isn't enough residual heat, put them back in a warm smoker for a while. Don't over do!
As far as time - anywhere from 45 minutes for pre-heated 300 degree oven with a few to 3 hours for cold smoker with 75! For three I'd figure on an hour or a little more just to be safe. You can hold them for quite a while in a heated cooler. Be sure to hold them unsliced, and slice right before serving.
Hope this helps.