Hi,
I cannot find SRX240-SMB2-CS-3 in the latest price list from juniper. Only the 1 year SKU as you already mentioned.
SRX240-SMB4-CS-3 is present though with the same list price so it should cover your needs.
(more specific: SRX240-SMB2-CS and SRX240-SMB4-CS has the same list price and I expect multiyear discounts to be the same).
I think SMB4 is the way to go - or look at a hardware refresh as the content security subscriptions are way cheaper on the new SRX300 series firewalls.
Eg. 3 years of content security bundle for SRX340 (SRX340-CS-BUN-3) has a list price of 2.700 USD. That's 74 USD cheaper than only oneyear for the SRX240 (SRX240-SMB4-CS-1). That can rather quickly justify new hardware :-)