None of these three players fit the mold of a traditional 'PF', but I would say they all do fit the mold of a modern 'wing'. So, in that sense, there's value in comparing them to see who might be the best of the three, despite all having very different styles of play.
Tatum is the most naturally gifted scorer, although Doncic isn't that far behind. Simmons is clearly lagging well behind in this skill, primarily because he just can't shoot from outside the paint.
Simmons is the most naturally gifted play-maker, although again, Doncic isn't that far behind in this regard. Tatum is clearly lagging well behind in this skill.
Simmons and Tatum seem to be about equal as rebounders, with Doncic a bit behind, but certainly capable of holding his own
Right now none of them are very good on defense. Simmons and Tatum probably have a bit higher upside here based on athleticism and length, but I wouldn't rule out Doncic, who seems to have the highest court awareness of any of these players. I do think Tatum will end up as the best defender of the bunch due to his combination of physical measureables, effort, and intellect. Simmons seems like the most likely to be perpetually indifferent on this end of the court.
All things considered, if I had to choose one of the three over the other, it would be really close between Tatum and Doncic, with Simmons a distant third. Tatum and Doncic look like they simply have more of that 'it factor', whereas Simmons seems content to just coast and I'm not sure I ever see him as the type to seriously work on his game.