Kumar Sangakkara did an excellent segment in which he explained why Bairstow's keeping technique pretty much makes it an impossibility for him to be a Test-class wicket-keeper, but that it's not as obvious because where a better keeper might get to a catch but drop it, Bairstow's body positioning means that he's not even getting to catches to his left hand side. This is particularly noticeable when you realise how often England's first slip fielder has to dive to their inside. Remember that speccy that Cook grabbed? That absolutely should have been Bairstow's catch.I think he's been quite reliable, actually. Although probably i wouldn't keep him at Buttler's expense. Probably sacrifice Foakes.
He also mentioned that much of Bairstow's training drills appear to be designed mostly to make him look good (diving catches and stuff) rather than actually making him a better keeper (improve his footwork so he doesn't have to dive as much).
If you're going to pick the best keeper of the three it has to be Foakes. There isn't any competition. But if you're going to pick the worst of the three, it's definitely Bairstow, which is convenient as Bairstow is also the best of the three in the outfield.
Unless you mean his batting, which has averaged under 30 over his last fifty innings, 48% of which he has been dismissed either bowled (34%) or LBW (14%), with seven ducks and a median score of 18.
In the same timeframe, both Jos Buttler and Ben Foakes have averaged above 40.