IMHO, it is not possible to use multiple fields like this in an action of the type "Format value".
You can only use special field names, like _ALL or _TAG (as you already know) to apply your adjustments to multiple fields at once.
will produce a single user-defined field %TITLE ARTIST ALBUM% with the contents of that what can be found in all the fields of the type COMPOSER, separated by a slash.
It would be helpful if you could explain what you want to achieve.
If I have to guess: the action does not distribute / copy the contents of COMPOSER to the field TITLE, ARTIST and ALBUM.
And even if it would do that - this would mean that you overwrite the existing contents of the 3 fields with that from COMPOSER. I don't know if that is your intention.
See this thread on a way how to apply a search/replace on several fields:
Please note that it is important to get a unique separator between the field vairables. This may be tricky as you want to get rid of more or less any ordinary punctuation character.