I think for next year the realistic expectations should be:
Betts - .280-.290 BA, .330-.340 OBP, ~.450 SLG, or a fringe .800 OPS player who adds value on the basepaths with about 20 SBs on the season at a better than 75% success rate, and finally above average to plus OF defense.
Bradley - .260-.270 BA, .330-.340 OBP, ~.400-.425 SLG, for a mid-700's OPS, solid base running that doesn't cost value and maybe adds a little, and elite defense.
Castillo - I think his current career line is actually fairly reasonable, .290 BA, .330 OBP, .435 SLG. Maybe a little less BA and a little more pop, s more like .285/.330/.445 or so, a higher end .700's OPS complimented by 10-15 SBs with only 3-4 CS and very good defense in RF.
If the Red Sox can get an aggregate OPS of .775 from these three guys and they all play 130+ games next year we'll see the OF go from a year and a half long problem to a source of strength, and that is before you factor in just how good they'll be together defensively.