Skylark

If you think of the role of DNA as solely to transmit genetic data, then I doubt a life form based on triple helix DNA would be that different if the triple helix could transmit the same amount of genetic data. I think the only difference between the double and triple helix would be in the energetic costs of DNA replication and translation. Presumably the triple helix would have a higher cost which would mean an organism using this type of DNA would need a higher metabolic rate to compensate. I would suspect this wouldn't make a dramatic change in the gross external morphology of the organism as warm-blooded tetrapods aren't that dramatically different from cold-blooded tetrapods relatively speaking even though their metabolic rates are many times greater.

Triple helix DNA occurs when a third oligonucleotide sequence occupies the major groove of double-stranded DNA.