I think all three of those guys have tremendous potential
A lot of the criticism of Monk is about his size and concerns on defense but he should be quick enough to guard opposing PGs since he's pretty athletic and quick. In today's NBA backcourt players need to be able to shoot and shoot well and Monk would certainly move us towards that. Remember the biggest criticisms of Steph Curry when he came into the league? "Good shooter but can't play PG and can't play defense". He's answered all of those (he's at least a mediocre defender and he can definitely play PG). He's 19 - 19-year-olds can always improve as playmakers depending on how well they're coached.
Smith is just an insane athlete. I see a poor man's Russell Westbrook. If he has a good motor (which I guess you won't see until the coaching staff actually talks to him), he could have the highest ceiling of the 3. There are some concerns about his knees but I guess that's up to our garbage medical staff.
As for Frank Ntilikina...he's an intriguing prospect. He has the physical tools to be a legit All-NBA defender. He's got a good motor, he's athletic enough (though he's not at Smith level of athleticism), and he can shoot. PGs with 7'0" wingspans are very rare and are the type of defenders who can really change a game on the defensive end. As much as we talk about needing playmaking and good PG play, let's not forget that defense was our biggest weakness last year (26th out of 30 in Defensive Rating) and that's probably our biggest need for improvement. We haven't had good backcourt defense in 20 years. I'm sick and tired of seeing guys like Lou Williams come in and torch us for 20+ points. I'm tired of the fact that every single opposing PG who comes into Madison Square Garden takes a dump on our team and has a career night. Ntilikina could go a long way in helping us improve on that end.
I think we would do great with any 3 of these players. Jonathan Isaac is also a great player but I don't see him falling to us.
As long as we don't draft Markkanen or Collins I'm good