Legal Question in Real Estate Law in California

Of course they can. If you are one day late they can. If you pay in the three days, they have to take it and can't evict you. If you don't, they can throw you out. It's no different for residential leases either, by the way. Any time the rent is late, a landlord can give a 3-day notice to pay or quit, and if rent is not paid in 3-days the lease is forfeited.

The landlord clearly can serve you with a three-day notice on the first day the rent is past due. Whether the landlord will follow up with an eviction process if you don't pay within the three days is less certain, but since there is a distinct possibility that it will, you should regard the notice as dead serious. If you need four, or five, days to scrape together the rent money, you should probably have a discussion with the landlord at once, to clarify your intent and learn more about its intent. Maybe this is just their way of handling overdue rent payments and they have no immediate plan to evict you, but there is a good possibility that they will follow through on the threat. Commercial landlords can have a variety of reasons for getting tough on late payments, including overall concerns about the tenant or wanting to clear the space for some big new tenant at a higher rent. This action also makes it likely that if you need to negotiate a renewal or extension, the terms you'll be offered won't be attractive.