1. The fun play will be put on in Room 1-3.
2. The fun play will be put on in Classroom 1-3.
The fun play will be put on in Class 1-3.
(Are they the same? Which expression do we have to use?)
It would be better not to use "fun" as a direct modifier. It's very slang-y! Use "entertaining" or "lively" or "witty" or something more specific:
http://thesaurus.com/browse/fun
Once you change "fun," all the sentences are OK, but #1 is the most commonly used.