It should be mentioned in https://www.zabbix.com/documentation/3.0/manual/config/items/itemtypes/snmptrap that snmptrapd must be started with '-On' for snmptt to work. That can be accomplished by adding
OPTIONS="-Lsd -p /var/run/snmptrapd.pid -On"
It's quite hard to find documentation about this tools. I believe this addition to the zabbix documentation will help many new users.