The goal is to have a sort of waterfall condition, with each selected value showing more results than the previous.

If selected value is 2,
   then show field B.
If selected value is 3,
   then show field B and field C.

Logically, the above sentence makes sense. However, building the condition in this way causes a problem. Specifically, a conflict is created in which field B is simultaneously shown and hidden. Because of this conflict, the conditions execute in order or precedence, which may not be the outcome expected by the user.

IF SELECT 2  | IF SELECT 3
THEN SHOW B  | THEN SHOW B
             |      SHOW C
ELSE HIDE B  | ELSE HIDE B
             |      HIDE C

The above example combines the show/hide logic into a single condition as a way of "stacking" (or water-falling) the conditional statements.

Revised:

If selected value is 2,
   then show field B [and hide field C].
If selected value is 3,
   then show field B and field C.

IF SELECT 2  | IF SELECT 3
       OR 3  |
THEN SHOW B  | THEN SHOW C
ELSE HIDE B  | ELSE HIDE C