I would go:
1) Siakam
2) Tatum
3) Simmons
For all Simmons strengths, the total absence of a perimeter game in today's NBA devalues him greatly. He can't shoot free throws, he can't even attempt 3s, and he commits a lot of turnovers.
Great thing for us as Celtics fans, Tatum is 4 years younger than Siakam, and almost 2 years younger than Simmons.