According to this article, Warner Bros have planned release dates for two unnamed event films in the next few years.
Judging by the release dates and trilogy status, it is almost safe to assume that these dates are for the subsequent Fantastic Beasts movies.
Fantastic Beasts 1 – November 18th 2016
Fantastic Beasts 2 – November 16th 2018 (unconfirmed)
Fantasic Beasts 3 – November 20th 2020 (unconfirmed)
This timeline gives a spacing of two years between each of the three movies.
Warner Bros has yet to confirm this.