I find it hard to vote for third year "rookies." Half the battle is adjusting to the NBA lifestyle and Embiid has been in the NBA for 2 years already. Very high potential though, so he'll win among this crop by default, especially with Simmons injury.
Otherwise, Brogdon and Siakam both have decent rookie PERs, play defense, and are playing >20 MPG currently. Siakam is doing it with the Raptors who are arguably the 3rd or 4th best team in the NBA which makes his contribution more impressive.