Your closest professional tandem dealers are:
Tandems Limited in Birmingham, AL (Jack & Susan Goertz)
Co-Motion, Bushnell, Bike Friday & Santana Triplets
Tandems, Touring & Recumbents in Greenville, SC (Scott McCrary)
Co-Motion & Santana Triplets + others as required.
Chain Reaction Cycles in Marysville, TN (Tim Patterson)
Co-Motion Triplets and tandems by Trek & Co-Motion
2408 E Lamar Alexander Pkwy
Maryville, TN 37804
The only dealers that probably "stock" new or used triplets are Precision Tandem (Mark Johnson) in Kansas, Mt.Airy Cyclery (Larry Black) in Maryland and Tandems East (Mel Kornbluh) in New Jersey. However, I sounds as if you already understand that a triplet is pretty much a near-custom bike.
Santana and Co-Motion probably have easily produced and sold more conventional triplets than anyone else, followed by Bike Friday's compact Family tandem triplets. We have friends here in Atlanta that have been riding one of Co-Motion's chromoly triplets for close to 8 years now. They've been very pleased with the bike. There was also a really nice Bushnell triplet in the area but I haven't seen that one in ages. I haven't seen any Santana multiseaters around here, but have seen several "Cabrio" models (very high-end take-a-part bikes that can be reconfigured as tandems, triplets, quads, etc...) at the regional rallies. As for comparing the quality and handling of a tandem to a triplet, you're really just left to compare the companies (or builder, in the case of Bushnell) based on reputation and cost to establish your best value as they are all comparable and all build bikes in both steel and aluminum. If cost is no object, Calfee and AriZona Tandems offer all-carbon triplets.
Co-Motion is a bit unique in that they now offer the very affordable 'Periscope' line of compact tandems that also includes a triplet. These frames are ideally suited for growing families or folks who like to hook up with other rides of varying size. At the Tennessee Tandem Rally Tim & Sharon Patterson teamed up with Traci Trumbull on a Periscope (probably pushing over 600lbs) and rode the Tail of the Dragon on the thing. This same bike has been used by several other combinations of some really strong riders to tackle Klingsman Dome and some other challenging rides here in the mid-South. These bikes warrant a close look-see.
Anyway, I'd recommend that you order up a catalog from Santana
just to bone up on their bikes. I don't think Dennis Bushnell
has a catalog, but you might call or Email to see. Tandems East is their biggest dealer, but Tandems Limited is now offering his bikes as well.
I would then call and talk to all three of the folks listed above. Any of these folks can sell you the same tandems: what you're looking for is good chemistry. These folks are all really nice and knowledgeable so find the right fit in terms of personality and willingness to meet your needs as a consumer. You might also want to give Mark Johnson @ Precision Tandems a call as well as Mel Kornbluh at Tandems East just to see what, if any, bikes they have in stock or what types of deals they may be able to offer. Buying long-distance ain't all that bad and, frankly, given Alabama's 9% sales tax, you'd probably want Tandems Limited to deliver your bike to you in Georgia anyway... South Carolina & Tennessee sales tax may be a bitter pill, but nothing comes close to the Alabama tandem tarriff.
Finally, there are a few second hand triplets that show up in the market place now and again. http://www.tandemmagazine.com/classified
is a good place to look and to post a "wanted to buy" for one of these beasts. There have been at least two triplets that I've wanted to buy, but since I really don't have any reason to own one I've reluctantly had to pass. Both were excellent values.