Is the Cribl supporting SNMPv3?

You can use the Exec source to pull via SNMP v3. https://docs.cribl.io/stream/sources-exec/

But then I have to have a snmptrap installed on worknodes, right?

Are you looking to send traps to Cribl or pull the SNMP values?

Sending traps to the Cribl.
Unfortunately installation additional packages on Cribl work nodes is not allowed

At this moment the Cribl is not supporting SNMPv3

Work is underway to be able to support SNMP v3