They are all different players.
Boldy is a winger. I don't even know if he's played center before, but definitely looks like a winger at the next level.
Zegras has the same types of positional problems as Hughes. Physically weak and somewhat of a perimeter game. I actually do like his defense and compete level. He's also slightly taller than Hughes and not as turnover prone, so he might even be more likely to stick at center than the better player Hughes, but its the same type of positional situation.
Turcotte is a definite center, and maybe an elite defensive one in the NHL eventually. He's a turnover-forcing machine. Absolute nightmare for defensemen who can't move the puck. He has a great read of the game defensively. He was first over the boards for the 17's NTDP team for a defensive zone shift late in the game or penalty kill. He's only 5'11, but he has a pro build. Really strong kid. Holds people off using his lower body strength. Goes to the dirty areas in all three zones.
Offensively, they are different, as well. Boldy plays a skill game, but not anywhere near what Turcotte or Zegras do. If he only played a skill game, he would be a second round pick or maybe third round pick, but the fact that he's a hard worker who competes really hard and has an excellent motor helps him out. He reminds me in a lot of ways of a smaller and not as good Brady Tkachuk. He plays with some skill, but a lot of effort. Probably only an okay skater, not going to dance his way through the whole defense for a highlight-reel goal, but he can keep up with skilled guys. Needs to put on weight for his type of game, but I can see him as a two way winger who scores 40 points per season in his prime leaning more towards a power forward than a finesse forward.
Zegras reminds me a lot of Mat Barzal. Elite speed breaking out of the zone, puck handling and playmaking. Definitely more of a playmaker than goal-scorer. Smart player who can control a shift. I like his defensive game, despite clearly needing to put on weight. He's very good, just overshadowed by the fact that two guys on his team also playing center are better players. I'd say 70-75 point guy per season who is competent, although not great, defensively is definitely possible.
Turcotte is a two way guy, but if he didn't have a two way game, he'd still be a very good offensive prospect. He's another great skater breaking out of the zone. He can get wherever he wants on the ice with his skating. Very good playmaker with a high hockey IQ. Definitely skilled, but he's not as skilled as Hughes. Hughes can legitimately walk a whole team on a shift, skate around the zone a couple of times and not break a sweat. Turcotte can walk players and take over shifts at a normal level for a very skilled offensive player. One area where he's not great is his shot. Its not terrible, but I'd say below-average. Not much of a shooter either. He has tremendous hands. Better than Zegras. Maybe as good as Hughes in this regard, so he scores a lot of goals in close. Projects as like a comfortable 75 point per season player with Selke potential.
Overall, Boldy is clearly the lesser of the three guys. He has potential to be around an average 4th winger, so thats not bad and he plays a safe enough game that he can play towards the bottom of your lineup if the rest of his game doesn't pan out offensively. Zegras and Turcotte are much better prospects. Zegras has 1C potential, but he has to first stick at the position and then emerge into a bigger role. Turcotte is not the tallest guy and he's not a huge goal-scorer. He's dynamic, but not going to be the most dynamic player in the NHL. He's a likely 1C as I see it. Probably not the top of the league 1C's. Somewhat around what Nico Hischier could be projected as.
I think there's separation here in how they'd rank. Boldy isn't bad, but at least a couple tiers behind the other two. There's also a gap between Turcotte and Zegras because Turcotte is going to be a better two way guy and the offense is probably very similar, even shaded towards Turcotte maybe. More proven, as well. Its a much smaller gap though between those two compared to Boldy. Its maybe five or so draft slots compared to about 20-25 when comparing those two to Boldy.
Ranking the three:
Turcotte-Definite top 5. Likely top top 3. Best case to a Hughes alternative at 1 is probably Turcotte.
Zegras-Probably stays within top half of first. Potential would be around #5 for his maximum slot.
(gap)
Boldy-Late first guy for now. Could get towards around #20 for maximum. Round 2 is also possible.