As of March 1, 2018, three-year SSL certificates will be discontinued. The new maximum lifespan will be two years. This is a CA/B Forum decision, not ours.
Encryption technology is evolving daily. Short lifespans mean less time between updates, which is safer.
The shorter the validity period, the less chance of a certificate being stolen or compromised.

You’ll still be able to carry over up to three months if you renew on time.
Important dates:
Feb. 20, 2018—Symantec stops accepting three-year orders
Mar. 1, 2018—Comodo stops accepting three-year orders