If your MOC has 3 open slots to the same SA and is submitting 3 slates this year, candidates in a future year may find themselves without a slot.
Nominating for 2 open slots happens every so many years due to the math of 5 slots over the course of 4 years. However, that same math means that, if you fill 3 vacancies (of 5) in one year, there will come a year that there are potentially no vacancies.
Just saying. . .
Hopefully, your MOC with 3 vacancies is nominating for 2 and holding one back for the year when he/she won't have a "natural" opening. BTW, for those who are curious, the main reason an MOC can have 3 slots is that one or more of the mids currently slotted to him/her left before graduation.