Your question is a bit cryptic. Generally speaking, a demurrer is not permitted in an unlawful detainer lawsuit. Moreover, improper service of the 3 day notice would be raised in the answer but would not be grounds for a demurrer.
The landlord can decide to proceed with the currently pending unlawful detainer trial. Or, the landlord can decide to dismiss the currently pending unlawful detainer and re-file a new complaint for unlawful detainer based upon the newly served 3 day notice to pay or quit.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This Avvo.com posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, please consult with your own attorney.