Update versions (otherwise you install a gem from source as beta3 but internally it's beta2) and update CHANGELOG.

activemodel/lib/active_model/version.rb

@@ -3,7 +3,7 @@ module ActiveModel
     MAJOR = 3
     MINOR = 0
     TINY  = 0
-    BUILD = "beta2"
+    BUILD = "beta3"
 
     STRING = [MAJOR, MINOR, TINY, BUILD].join('.')
   end