From the Zabbix Manual the calculated items expression follows the form,
func(<key>|<hostname:key>,<parameter1>,<parameter2>,...)
This is fine for computations using functions over a single item like,
max("temp1",120)
How should a function like min() be applied over 3 different items so it returns the lowest of those?
Unfortunately, that is currently not supported. You may vote on a feature request: https://support.zabbix.com/browse/ZBXNEXT-1829