Here is a link to Chuck's Bikes bicycle frame page:
http://www.chucksbikes.com/frames.htm
You'll have to scroll through to find the tandems. These frames are just about all from Asia. The tandems seem to come from Kinesis (KHS & several European models), ORA Engineering (who manufactures frames for Cervelo, Schwinn/Diamondback, Raleigh & others) and one or two others. Quality is spotty. One of the triplets -- from ORA -- was purchased by a tandem dealer/friend in Kansas and it was really rough. Even as someone who makes a second career living out of selling and working on tandems he had a hard time getting this one put together and may still be working on it. Definitely not a lot of Quality Control. But, this was just one and other off-brand name tandems (Scott & Look) as well as Chuck's own Tsunami-branded tandems have been sold that seem like OK frames for entry level - intermediate use.
Somebody mentioned a triplet as a possible addition to their fleet. Perhaps one of the best deals I've seen on an adult-size triplet is from St. John's Street in the UK: the Thorn Trident at £899 (~$1,450 USD). Shipping would add about $200 but that's still a deal on a complete, ready to ride triplet. It weighs about 80lbs and is made in Asia for Thorn but it's as durable as can be and up to the task of hard riding. We have some very strong riding partners who own one and they've done MS150's on it with 3 adults and hammered along with just about everyone else on the flats and downhills, topping out at speeds in the 50mph range with no worries about feeble equipment.
For kids, Thorn also makes a Me-N-U2 triplet sized for an adult and two children for about $1,950. This is a somewhat better quality bike that is made by Thorn.
http://www.sjscycles.com/26tandembro...churewebp3.htm
Good hunting.