DowDuPont Inc., created from the world’s biggest chemical merger, boosted the cost savings it expects to achieve and accelerated its planned split into three independent companies.

The separations will be completed by June 1, 2019, DowDuPont said in a statement Feb. 1. That’s three months earlier than a forecast in October, when the company had disappointed some investors by extending its original timeline.

The materials-science business will go it alone by the end of March 2019, with the agriculture and...