Atypical DNA and RNA secondary structures play a crucial role in simple sequence repeat (SSR) diseases, which are associated with a class of neurological and neuromuscular disorders known as "anticipation diseases," where the age of disease onset decreases and the severity of the disease is increased as the intergenerational expansion of the SSR increases. While the mechanisms underlying these diseases are complex and remain elusive, there is a consensus that stable, non-B-DNA atypical secondary structures play an important - if not causative - role. These structures include single-stranded DNA loops and hairpins, G-quartets, Z-DNA, triplex nucleic acid structures, and others. While all of these structures are of interest, structures based on nucleic acid triplexes have recently garnered increased attention as they have been implicated in gene regulation, gene repair, and gene engineering. Our work here focuses on the construction of DNA triplexes and RNA/DNA hybrids formed from GAA/TTC trinucleotide repeats, which underlie Friedreich's ataxia. While there is some software, such as the Discovery Studio Visualizer, that can aid in the initial construction of DNA triple helices, the only option for the triple helix is constrained to be that of an antiparallel pyrimidine for the third strand. In this protocol, we illustrate how to build up more generalized DNA triplexes and DNA/RNA mixed hybrids. We make use of both the Discovery Studio Visualizer and the AMBER simulation package to construct the initial triplexes. Using the steps outlined here, one can - in principle - build up any triple nucleic acid helix with a desired sequence for large-scale molecular dynamics simulation studies.