Using anonymize filter to add checksum.

logstash.conf

# Add field which is composed of multiple source fields.
  
filter {  
  mutate {
    add_field => [ "hash", "%{field1}/%{field2}/%{field3}" ]
  }
  anonymize {
    algorithm => "SHA1"
    fields => [ "hash" ]
    key => "keyvalue"
  }
}