In addition to the good advice above, the training plan can also be affected by where the triathlete is in their season (off-season, start, mid, etc) and how competitive they are.
If you listen to Frazier Quelch, former triathlete and head of training and development for TRX, the most messed up his body was was when he was a competitive triathlete. While a serious athlete might be willing to sacrifice their body for sports excellence, others may not. Hence my comment on how competitive they are and whether they are.
The only two triathletes I’ve worked with (one had done 6 iron mans and her husband had done one) I worked with them off-season. I put them in the frontal and transverse planes to support the directions that were less efficient.