Hi George,
Thanks for posting at the new 3DNA forum!
One way to build a standard DNA triplex with a mixture of TAT and CGC(+) triples would be first to build a Poly (U) : poly (A) : poly(U) triplex, using 
fiber model #32, or another one as you see fit. Run 
fiber -l for a list of possible fiber models available from 3DNA, and read the 3DNA 2003 NAR paper for details.
Next, you can use 
mutate_bases to mutate A/U to the bases you need. See the 
documentation for details -- currently 
mutate_bases is the only program I have documented.
Third, you need to 
download 3DNA v2.1beta which has 
mutate_bases incorporated as an integral part of the new distribution.
Note that 3DNA v2.1 is only in 
beta test version. If you notice any issues, please report back.
Best regards,
Xiang-Jun