Can I use field validation to turn a field “on” or “off” based on the selection in a pick list field

Example:

1. Field A is a pick list with 5 values.
2. Field B is present on the layout but grayed out (read only).
3. When the user selects pick list value 3 from field A, field B is activated and lets the user input further info.

Note: Pick list value 3 is the only value that activates field B, all other values are ignored by the expression.

Expression tried:

([<IndexedPick2>] LIKE "A TERMINO FIJO") OR ([<stDuracin_ITAG>] IS NULL)

This is not working.