BIP39 mnemonics are restricted to encode 128 to 256 bits of entropy corresponding to 12 to 24 words. Since 128 bits of security already appears to be quite enough (see this answer on Information Security StackExchange), a better question might be why it even allows you to go up to 256. However I can't imagine you could need even more than that, because if the cryptography behind mnemonics is so broken in the future that 24 words aren't enough, adding more words probably won't help.
Other than setting reasonable limits, there's nothing in BIP39 preventing you from using any number of words that is a multiple of 3 (including 3 or even 0). You could code your own wallet, or better yet you could modify an existing wallet's source code to remove these limits. However I would advise not to store any actual funds with this mnemonic seed. The standard exists so that a seed can be used with any compatible wallet, and using an incompatible seed defeats that purpose.
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