Received 3 checks this month from CA with 2018 and current date. Why send me money from 2018 and why in 3 installments? 2 checks cleared, just got 3rd. ok to cash?

No one in this user forum would know about checks sent to you by the state.   Contact the state to ask them why the checks were sent---or if it is some sort of scam.

https://ttlc.intuit.com/community/state-taxes/help/how-do-i-contact-my-state-department-of-revenue/00/25573