Neil Walker, Dave Davidson and Brian Slocum have all been sent to Class AAA Indianapolis. Walker had to go because keeping him in camp would have started his service clock, but it's moot anyway--Walker isn't ready for the big leagues, and no amount of spring hitting would've really proven otherwise. He's still a prospect, but he needs to hit well at AAA this year to be taken seriously as a starting big-league infield option.
As for the other two, no one really knows what Davidson is still doing on the 40-man roster at this point, and Slocum is AAA roster filler. Neither had any shot at a big-league job.