We have build a couple of triplets already. Some of them with adjustable child bottom bracket which “grows” with the child. This bike has to be configured in close discussion with future owners as we need to know the way they will use it.
You really have to know why you want such a bike, and previous experience with tandem bikes is an advantage. If you do not have it, try to rent a tandem somewhere for a couple of days and try it first. Then we will have a chance to prepare the specification exactly for your use.
This bike doesn`t allow that many combinations, and riders should sit on the bike from the the tallest one to the smallest one in order to make the frame nice and stiff. The only exception is when you ride with your child, which can sit in the middle and can pedal using adjustable bottom bracket. You can see such configuration on the main picture.
We can build also a version where you can split the bike into three parts and make only a tandem out of it. But this is almost a “custom project” for winter time.