There are several variants of this, "24/7", "24-7", "24/365", "24X365" and so on.
Those with a multiplication are clearly depending on the fact that e.g.
24×7 will indeed, if calculated, give the number of hours in a week.
The main point of that expression is not to convey 168 nor for "24×365" to convey 8760 (which forgets leap years anyway). It's to convey that information which is also available in the forms which don't use the multiplication symbol, viz that the service, shop, etc. is available at every hour and at every day of the week and/or year.
"24-7-365" conveys availability at every hour and at every day of the week and at every day of the year. The "7" is redundant when we have the "365" but it relates to a different concern: It might be more important to me that a service is available on Saturdays than that it is available on Christmas, so I might care about the "7" more than I do about the "365" even though the latter logically entails the former.
The mathematically incorrect "24×7×365" is just a merging of that with the forms where the arithmetic actually does work, because similar common expressions will always end up being merged, especially when written quickly.