Here's why every three may not work.
You start with the following points per use year (let's say you have 50):
2017 50
2018 50
2019 50
You want to go in 2018 and you want a week in October at BLT Theme Park view: it costs 151 points. Even if you borrow all points from 2019 you are 1 point short. You cannot book the whole reservation at 11 months, but you can purchase a one time use point from MF, but that can be done only at 7 months.
So at 11 months you can only book 6 days and hope to be able to add the last night at 7 months. Here's what you would have at 11 months:
2017 0
2018 128 (used for 6 nights reservation)
2019 22 (leftover, hoping to be able to combine them at 7 months with one One Time Use point)
At 7 months you call but the extra night is not available (and having to call vs online booking increases the chances this could happen). I'm not sure if MF will allow you to buy one point to be able to waitlist (I think not), but let's say you're not lucky anyway and the wailist never comes through and you cannot secure the last night.
You now have 22 points in the 2019 UY that you cannot use and you must rent or loose them.
In fact now you would have the following points:
2019 22
2020 50
2021 50
2022 50
If you go every three years, your next vacation would be 2021, but points can be banked only once, so the 22 points can be used only in 2020, not in 2021. You could go in 2020, but you would have only 122 point, not enough even for 6 nights and trouble starts all over again.
Moreover, for your 2018 trip, if you need to cancel, points cannot be put back in their UY and one time use points cannot be banked, so if you have to cancel you will have to rent the points during 2017 UY or loose them.
Going every three years is possible, but it comes with higher risks and requires more flexibility (for example, you could book the last night in a different resort and do a split stay, change dates, change resort...).