I dunno I think all the guys in the top 3 will have some kind of impact in year 1
It's not guaranteed. They'll have impact because highly drafted players will likely play a lot of minutes barring injury, but take a look at top 5 picks in 2014 and 2013.
2013:
1- Anthony Bennett - Cleveland - Bust
2- Victor Oladipo - Orlando - Good
3- Otto Porter - Washington - very minor impact in year 1, below average contribution in year 2.
4- Cody Zeller - Charlotte - Low impact.
5- Alex Len - Phoenix - very minor impact in year 1, below average contribution in year 2.
2014:
1- Andrew Wiggins - Minnesota - Good
2- Jabari Parker - Milwaukee - Injured, no impact.
3- Joel Embiid - Philadelphia - Injured, no impact.
4- Aaron Gordon - Orlando - low impact.
5- Dante Exum - Utah - low impact.
That's 2 impactful/good players from the top 5 in the last 2 drafts. There's still time for many of these players to show more, it's still too early, and some were injured, but the point is draftees don't generally have big impacts in their first/second year, barring those really special ones.
Now I do think the top 3 picks in this draft could potentially be all really good NBA players, even maybe all three reach all-star caliber, but I would not be surprised if their impact is not great in their first season.
Also, big men have a tougher time adjusting, so expect both Towns and Okafor to struggle a bit early. Russell might have the best first season of the three.