Hi gvandaal,
Restricting access to the certain IP and/or subnet in [snmp] config stanza is not present for snmpv3 afaik.
But you always can use control plane protection filter for lo0.0 interface.
Just create a rule to allow snmp from your monitoring system IPs. For example:
set policy-options prefix-list SNMP 10.1.1.1/32
set policy-options prefix-list SNMP 192.168.1.0/24
set firewall family inet filter CoPP term SNMP from source-prefix-list SNMP
set firewall family inet filter CoPP term SNMP from protocol udp
set firewall family inet filter CoPP term SNMP from destination-port snmp
set firewall family inet filter CoPP term SNMP then accept
set firewall family inet filter CoPP term SNMP then count SNMP
Just don't forget to allow mgmt traffic to your box.
You can find more details on protecting control plane of your Juniper routers in the "Day One: Securing the Routing Engine" book.
Thanks,
Alex