In this situation, I don't believe we are in the position to guess on the dosing intent of the prescriber. It would make sense to contact the prescriber and get clarification on the directions to eliminate any ambiguity. If you are unclear on any sort of medical directive, it is always better to get clarification than to guess.
It is important when dosing Fleet enema products, which are also called "saline laxatives", that they are used "as needed", per the doctors instructions, and not continuously every third day if treatment is not necessary.
In terms of product labeling, the only time related contraindication is to not give more than one Fleet enema in a 24 hour period. This is due to the potential serious adverse effects that can come from the overuse of saline laxatives.
In 2014, FDA Medwatch released a safety announcement regading all saline laxatives. Overuse is associated with rare but serious cases of kidney failure, electrolyte imbalances and severe dehydration.
Around the same time as the 2014 FDA announcement, Fleet (the manufacturer) released a product poster to highlight the risks of using Fleet laxatives incorrectly.
Having said all of the above, there certainly are situations and patients that may require more continuous dosing of saline laxatives. It is important to watch and monitor for potential adverse reactions and signs of dehydration. Deviating from the directions of "every 72 hours" by a few hours once in awhile should be relatively insignificant. It is more important that the product is used only "as needed" in that general time frame. As mentioned, you should speak to the prescriber to get clarification on exactly what they expect from those administering the drug.