I get what you're trying to say, but really those three aren't close to the three the Sox gave up. And each team is going to have different priorities on return (Near ready vs lotto tickets).
Leiter at 2.5 years of control 1.3 fWAR this season (1.9 total counting last season) > Kopech 1.5 years of control -.2 fWAR this season (-1.1 counting last season)
Tauchman (104 wRC+) 2.5 years of control vs 36 year old rental Pham (102 wRC+)
And then Fedde vs Taillon. I'm not as high on Fedde as it appears most folks in the twitter world is. He was basically worth a little over 1 fWAR his previous 6 year big league stint. He did make changes after his year in the KBO and so far he's been solid (xERA 3.77, xFIP 3.96). Do I buy he's that guy again next year when he currently doesn't get a ton of whiffs or chase, no I really don't. Especially if you factor in a more bullish offensive environment next year.
Part of me thinks they could of done a little better in their prospect haul, but also part of me thinks they just turned a short term project who they signed from the KBO into 5/6 years (I don't know how much control Vargas has since he's been up and down a few times) of Miguel Vargas and two ascending young prospects from the Dodgers farm who probably slot in close to the Sox top 10.
Here's to hoping there's regression coming Fedde's way.