Here's Tatum vs Brown vs Hayward based on this year (and last for Hayward):
http://bkref.com/tiny/0Tz0xFocus on stats per 100 possessions and advanced stats to equalize.
Rebounds: even
Assists: Hayward by a landslide
Points: Hayward by a landslide
Free throws: Hayward by a landslide (makes), also %
FG/3PT%: pretty close across the board
TS%: Hayward
Turnovers: Hayward lowest rate on highest usage
+/-: Hayward by a landslide
STL: even
BLK: Hayward last
Tatum & Brown will continue to improve, and Hayward is obviously coming back from a major injury, but in terms of NBA games already played Hayward has reached a higher offensive level. Tatum has the highest offensive ceiling IMO but he may not surpass Hayward's peak for another couple years.