Originally Posted by
sandyph
...so theoretically, you can get 3 years Plat for this year
No, you will only get credit for two years at status. From members.Marriott:
If you achieved Elite status in both Rewards and SPG in the same year, you’ll receive credit for two Elite years.
Of course, anyone who gets status in both legacy programs would, by definition, also have status when combining nights for the new program, but it is obvious from the wording that the intent isn't to count the combined status as a unique year. The nights will not count twice. You cannot receive credit for three Elite years, only two.