Legal Question in Real Estate Law in California

You cannot rely on an internet Q&A for interpretation of a document, whether it is a lease or anything else. A document as a whole must be read together before an opinion can be given about it's effect. The tenants may also assert oral modifications or side agreements, and if the landlord accepts rent for any time beyond the end of the lease that too will extend the tenancy. But the simple base law is that when a lease expires, if the landlord elects not to renew or extend and the property is not in a "just cause eviction" jurisdiction, the tenancy ends and if they tenants do not move out they are liable for unlawful detainer. The landlord may then sue for eviction and unlawful detainer damages, just as if a tenant failed to pay rent.

A 90-day lease is rather unusual, and you don't say that it is a residential lease, nor can we be sure where the leased property is and therefore whether it is subject to any rent-control or other tenant protection laws. Nevertheless, I'd also go along with Mr. McCormick's conclusion that you are free not to renew the lease and to terminate and, if necessary, evict the tenants.