All three have tremendous upside. The problem is that there is still a long way for them to develop into good baseball players and there is no guarantees that any of them will reach their potential. Because they have a long way ahead of them, it's tough to project what kind of player each one of them will be. Moreover, not a whole lot of people have seen these guys play, so we have to rely on the opinions of a few people, most of whom are working with the organization that signed the player, which means we are going to get a lot of "The best international player we have ever seen".
I think the best source of info right now on these guys is probably from Baseball America.
This is from BA from the time Juan Duran was signed. Link
...Duran, who bats and throws righthanded and checks in at roughly 6-foot-6, 190 pounds, likely will not play a game this year in the PL, however, where at 16 years old he would be by far the youngest player in the league. The Reds cannot assign him to the Gulf Coast League, since the GCL season ends on Aug. 27, so sending him to the GCL would void his contract.
Buckley and Arias were both with the Blue Jays when the team drafted outfielder Alex Rios as an 18-year-old with the 19th overall pick in the 1999 draft.
"He compares favorably, only he's bigger and stronger at this age than Rios was," Buckley said. "He's more physical, and he has shown some of the same feel for hitting that Alex displayed.
"I've never seen a 16-year-old with this type of ability. I talked to some people in the Dominican Republic who crossed paths with Vladimir Guerrero at that age, guys who have been scouting in the Dominican for more than 20 years, and they said they've never come into contact with a bat like this. He's got 80 power potential (on the 20-80 scouting scale) and just gets tremendous loft. In my opinion, in terms of his bat and his body, he's a better overall package than Angel Villalona. We wanted to change the perception in the baseball industry that Cincinnati is a cheap organization. We want to get back to being the Big Red Machine."
Another AL international scouting director said his team was also interested in Duran.
"We liked him," another AL international scouting director said. "We saw him as a corner outfielder, big, long-limbed, real bat potential. He's definitely a guy a lot of people liked."
One National League international scouting director compared Duran's body to that of a young Juan Gonzalez.
"He might end up being in center field because he's a plus runner. It's all going to come down to how big he gets and if he slows down," the scout said. "He's going to put on some weight, so whether he retains that speed will tell if he ends up at a corner.
"He's got some bat speed, and the power will come from the bat speed, but it all comes down to how thick he grows."
Here is what BA had to say about Rafael Rodriguez. Link
At 6-foot-5, 198 pounds, Rodriguez has good raw power and a projectable hitter's frame. A righthanded batter, Rodriguez has plenty of tools—he's an above-average runner with an above-average arm—but some scouts weren't sold on his hitting mechanics and overall feel for hitting.
"From a tools perspective," said one international scouting director, "you're not gonna find a better kid."
A little bit more of info can be found on Michael Almanzar than the other guys because he was signed last year and has already spent some time on the Gulf Coast League and the South Atlantic League. This is from Kevin Goldstein from Baseball Prospectus at the end of June when he was still at the GCL.
Almanzar was one of the big prizes in last year’s international signing period. The Red Sox spent mid-first-round money ($1.5 million) on the Dominican slugger, betting on his excellent swing mechanics and tons of physical projection. Making his pro debut in the Florida complex league, Almanzar began his career with four consecutive two-hit games, and extended his hitting streak to eight games by reaching base in all five of his plate appearances on Sunday, going 3-for-3 with a triple and a pair of walks. Now batting .412/.474/.500, Almanzar is having little difficulty hitting, while on defense the reports are "so far, so good" on his transition from shortstop to third base. It’s far too early to call, but Almanzar’s talent is looking like it was properly valued by Boston.
Almanzar then moved up to SAL and struggled a little, but I wouldn't read too much into his numbers at SAL.
Hope this helps.